diff --git a/.circleci/config.yml b/.circleci/config.yml
index ef6b7ce80..ce623b054 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -67,7 +67,7 @@ jobs:
       name: golang
     steps:
       - run: '[[ -n $CIRCLE_PULL_REQUEST ]] || circleci step halt || true' # Skip if this is not a pull request
-      - run: curl https://moleculacorp:$GITHUB_PERSONAL_ACCESS_TOKEN@api.github.com/repos/molecula/pilosa/pulls/$(basename $CIRCLE_PULL_REQUEST) | jq "[.labels[] | .name | startswith(\"changelog\")] | any" -e
+      - run: curl https://moleculacorp:$GITHUB_PERSONAL_ACCESS_TOKEN@api.github.com/repos/molecula/featurebase/pulls/$(basename $CIRCLE_PULL_REQUEST) | jq "[.labels[] | .name | startswith(\"changelog\")] | any" -e
   test-build-arm:
     executor:
       name: golang
diff --git a/api.go b/api.go
index d2c09fe85..a0074371c 100644
--- a/api.go
+++ b/api.go
@@ -2179,6 +2179,263 @@ func (api *API) TranslateIndexDB(ctx context.Context, indexName string, partitio
 	_, err := store.ReadFrom(rd)
 	return err
 }
+func (api *API) mutexCheckThisNode(ctx context.Context, qcx *Qcx, indexName string, fieldName string) (map[uint64]map[uint64][]uint64, error) {
+	index := api.holder.Index(indexName)
+	if index == nil {
+		return nil, newNotFoundError(ErrIndexNotFound, indexName)
+	}
+	field := index.Field(fieldName)
+	if field == nil {
+		return nil, newNotFoundError(ErrFieldNotFound, fieldName)
+	}
+	return field.MutexCheck(ctx, qcx)
+}
+
+// mergeIDLists merges a list of numeric IDs into another list, removing
+// duplicates.
+func mergeIDLists(dst []uint64, src []uint64) []uint64 {
+	dst = append(dst, src...)
+	sort.Slice(dst, func(i, j int) bool {
+		return dst[i] < dst[j]
+	})
+	// dedup.
+	n := 0
+	prev := dst[0]
+	for i := 0; i < len(dst); i++ {
+		if dst[i] != prev {
+			dst[n] = dst[i]
+			n++
+		}
+		prev = dst[i]
+	}
+	return dst
+}
+
+// mergeKeyLists merges a list of string IDs into another list, removing
+// duplicates.
+func mergeKeyLists(dst []string, src []string) []string {
+	dst = append(dst, src...)
+	sort.Slice(dst, func(i, j int) bool {
+		return dst[i] < dst[j]
+	})
+	// dedup.
+	n := 0
+	prev := dst[0]
+	for i := 0; i < len(dst); i++ {
+		if dst[i] != prev {
+			dst[n] = dst[i]
+			n++
+		}
+		prev = dst[i]
+	}
+	return dst
+}
+
+// MutexCheckNode checks for collisions in a given mutex field. The response is
+// a map[shard]map[column]values, not translated.
+func (api *API) MutexCheckNode(ctx context.Context, qcx *Qcx, indexName string, fieldName string) (map[uint64]map[uint64][]uint64, error) {
+	if err := api.validate(apiMutexCheck); err != nil {
+		return nil, errors.Wrap(err, "validating api method")
+	}
+	return api.mutexCheckThisNode(ctx, qcx, indexName, fieldName)
+}
+
+// MutexCheck checks a named field for mutex violations, returning a
+// map of record IDs to values for records that have multiple values in the
+// field. The return will be one of:
+//     map[uint64][]uint64 // unkeyed index, unkeyed field
+//     map[uint64][]string // unkeyed index, keyed field
+//     map[string][]uint64 // keyed index, unkeyed field
+//     map[string][]string // keyed index, keyed field
+func (api *API) MutexCheck(ctx context.Context, qcx *Qcx, indexName string, fieldName string) (result interface{}, err error) {
+	if err = api.validate(apiMutexCheck); err != nil {
+		return nil, errors.Wrap(err, "validating api method")
+	}
+	index, err := api.Index(ctx, indexName)
+	if err != nil {
+		return nil, err
+	}
+	field, err := api.Field(ctx, indexName, fieldName)
+	if err != nil {
+		return nil, err
+	}
+	if field.Type() != FieldTypeMutex {
+		return nil, errors.New("can only check mutex state for mutex fields")
+	}
+	nodes := Nodes(api.cluster.nodes).Clone()
+	eg, _ := errgroup.WithContext(ctx)
+
+	results := make([]map[uint64]map[uint64][]uint64, len(nodes))
+	myID := api.Node().ID
+	//	vprint.VV("MyID %#v\n", myID)
+	for i, node := range nodes {
+		i := i // loop variable shadowing is a war crime
+		//		vprint.VV("Compare %#v with %v", node.ID, myID)
+		if node.ID != myID {
+			node := node // loop variable shadowing again
+			eg.Go(func() (err error) {
+				results[i], err = api.server.defaultClient.MutexCheck(ctx, &node.URI, indexName, fieldName)
+				return err
+			})
+		} else {
+			eg.Go(func() (err error) {
+				results[i], err = api.mutexCheckThisNode(ctx, qcx, indexName, fieldName)
+				return err
+			})
+		}
+	}
+	err = eg.Wait()
+	if err != nil {
+		return nil, err
+	}
+	// We now have a series of maps from shards to maps of record IDs to
+	// values. But wait! Either the field, or the index, might be using keys,
+	// and want those translated. So we have to translate those.
+	useIndexKeys := index.Keys()
+	useFieldKeys := field.Keys()
+	var indexKeys = map[uint64]string{}
+	var fieldKeys = map[uint64]string{}
+	var indexIDs []uint64
+	var fieldIDs []uint64
+	// build translation tables, if we need them
+	untranslated := "untranslated"
+	// We don't know which of four map types we want to be working with,
+	// but what we can do is make a function which works with that map type
+	// given the raw integer values, and is a closure with an already-created
+	// map which has already been stashed in `result`. Because maps are
+	// reference-y, this should actually work.
+	var process func(uint64, []uint64)
+	if useIndexKeys || useFieldKeys {
+		for _, nodeResults := range results {
+			for _, shardResults := range nodeResults {
+				for record, values := range shardResults {
+					if useIndexKeys {
+						if _, ok := indexKeys[record]; !ok {
+							indexKeys[record] = untranslated
+							indexIDs = append(indexIDs, record)
+						}
+					}
+					if useFieldKeys {
+						for _, value := range values {
+							if _, ok := fieldKeys[value]; !ok {
+								fieldKeys[value] = untranslated
+								fieldIDs = append(fieldIDs, value)
+							}
+						}
+					}
+				}
+			}
+		}
+		// we now have lists of the keys, so...
+		if useIndexKeys {
+			indexKeyList, err := api.cluster.translateIndexIDs(ctx, indexName, indexIDs)
+			if err != nil {
+				return nil, errors.Wrap(err, "translating index keys")
+			}
+			if len(indexKeyList) != len(indexIDs) {
+				return nil, fmt.Errorf("translating %d record IDs, got %d keys", len(indexIDs), len(indexKeyList))
+			}
+			for i := range indexIDs {
+				indexKeys[indexIDs[i]] = indexKeyList[i]
+			}
+		}
+		if useFieldKeys {
+			fieldKeyList, err := api.cluster.translateFieldListIDs(field, fieldIDs)
+			if err != nil {
+				return nil, errors.Wrap(err, "translating index keys")
+			}
+			if len(fieldKeyList) != len(fieldIDs) {
+				return nil, fmt.Errorf("translating %d IDs, got %d keys", len(indexIDs), len(fieldKeyList))
+			}
+			for i := range fieldIDs {
+				fieldKeys[fieldIDs[i]] = fieldKeyList[i]
+			}
+		}
+
+	}
+	// define the process functions. separated from above code just to make
+	// it easier to follow/compare them.
+	if useIndexKeys {
+		if useFieldKeys {
+			outMap := make(map[string][]string)
+			var valueKeys []string
+			result = outMap
+			process = func(recordID uint64, valueIDs []uint64) {
+				valueKeys = valueKeys[:0]
+				for _, id := range valueIDs {
+					valueKeys = append(valueKeys, fieldKeys[id])
+				}
+				record := indexKeys[recordID]
+				if existing, ok := outMap[record]; ok {
+					outMap[record] = mergeKeyLists(existing, valueKeys)
+				} else {
+					// The append is so we can reuse this buffer safely,
+					// which matters if there's replication, because many
+					// cases won't need to copy the buffer, they'll just
+					// copy individual things from it.
+					outMap[record] = append([]string{}, valueKeys...)
+				}
+			}
+		} else {
+			outMap := make(map[string][]uint64)
+			result = outMap
+			process = func(recordID uint64, values []uint64) {
+				record := indexKeys[recordID]
+				if existing, ok := outMap[record]; ok {
+					outMap[record] = mergeIDLists(existing, values)
+				} else {
+					outMap[record] = values
+				}
+			}
+		}
+	} else {
+		if useFieldKeys {
+			outMap := make(map[uint64][]string)
+			var valueKeys []string
+			result = outMap
+			process = func(record uint64, valueIDs []uint64) {
+				valueKeys = valueKeys[:0]
+				for _, id := range valueIDs {
+					valueKeys = append(valueKeys, fieldKeys[id])
+				}
+				if existing, ok := outMap[record]; ok {
+					outMap[record] = mergeKeyLists(existing, valueKeys)
+				} else {
+					// The append is so we can reuse this buffer safely,
+					// which matters if there's replication, because many
+					// cases won't need to copy the buffer, they'll just
+					// copy individual things from it.
+					outMap[record] = append([]string{}, valueKeys...)
+				}
+			}
+		} else {
+			outMap := make(map[uint64][]uint64)
+			result = outMap
+			process = func(record uint64, values []uint64) {
+				if existing, ok := outMap[record]; ok {
+					outMap[record] = mergeIDLists(existing, values)
+				} else {
+					outMap[record] = values
+				}
+			}
+		}
+	}
+
+	for _, nodeResults := range results {
+		if len(nodeResults) == 0 {
+			continue
+		}
+		for _, v := range nodeResults {
+			if len(v) == 0 {
+				continue
+			}
+			for record, values := range v {
+				process(record, values)
+			}
+		}
+	}
+	return result, nil
+}
 
 // TranslateFieldDB is an internal function to load the field keys database
 func (api *API) TranslateFieldDB(ctx context.Context, indexName, fieldName string, rd io.Reader) error {
@@ -2249,11 +2506,13 @@ const (
 	apiIDReserve
 	apiIDCommit
 	apiIDReset
+	apiMutexCheck //Backported from 1681 Caution
 )
 
 var methodsCommon = map[apiMethod]struct{}{
 	apiClusterMessage: {},
 	apiSetCoordinator: {},
+	apiMutexCheck:     {},
 }
 
 var methodsResizing = map[apiMethod]struct{}{
diff --git a/api_test.go b/api_test.go
index 3f6369476..bec6c8b1c 100644
--- a/api_test.go
+++ b/api_test.go
@@ -28,6 +28,7 @@ import (
 	"github.com/pilosa/pilosa/v2/boltdb"
 	"github.com/pilosa/pilosa/v2/http"
 	"github.com/pilosa/pilosa/v2/server"
+	"github.com/pilosa/pilosa/v2/shardwidth"
 	"github.com/pilosa/pilosa/v2/test"
 )
 
@@ -622,3 +623,317 @@ func TestAPI_ClearFlagForImportAndImportValues(t *testing.T) {
 		panic(fmt.Sprintf("expected %v, observed %v starting acct0 balance", acct0bal, 0))
 	}
 }
+
+type mutexCheckIndex struct {
+	index     *pilosa.Index
+	indexName string
+	createdAt int64
+	fields    map[bool]mutexCheckField
+}
+
+type mutexCheckField struct {
+	fieldName string
+	field     *pilosa.Field
+	createdAt int64
+}
+
+func TestAPI_MutexCheck(t *testing.T) {
+	c := test.MustRunCluster(t, 3)
+	defer c.Close()
+
+	m0 := c.GetNode(0)
+	nodesByID := make(map[string]*test.Command, 3)
+	qcxsByID := make(map[string]*pilosa.Qcx, 3)
+	for i := 0; i < 3; i++ {
+		node := c.GetNode(i)
+		id := node.API.Node().ID
+		nodesByID[id] = node
+	}
+
+	indexes := make(map[bool]mutexCheckIndex)
+
+	ctx := context.Background()
+	for _, keyedIndex := range []bool{false, true} {
+		indexName := fmt.Sprintf("i%t", keyedIndex)
+		index, err := m0.API.CreateIndex(ctx, indexName, pilosa.IndexOptions{Keys: keyedIndex, TrackExistence: true})
+		if err != nil {
+			t.Fatalf("creating index: %v", err)
+		}
+		if index.CreatedAt() == 0 {
+			t.Fatal("index createdAt is empty")
+		}
+		indexData := mutexCheckIndex{indexName: indexName, index: index, fields: make(map[bool]mutexCheckField), createdAt: index.CreatedAt()}
+		for _, keyedField := range []bool{false, true} {
+			fieldName := fmt.Sprintf("f%t", keyedField)
+			var field *pilosa.Field
+			if keyedField {
+				field, err = m0.API.CreateField(ctx, indexName, fieldName, pilosa.OptFieldTypeMutex(pilosa.CacheTypeNone, 0), pilosa.OptFieldKeys())
+			} else {
+				field, err = m0.API.CreateField(ctx, indexName, fieldName, pilosa.OptFieldTypeMutex(pilosa.CacheTypeNone, 0))
+			}
+			if err != nil {
+				t.Fatalf("creating field: %v", err)
+			}
+			if field.CreatedAt() == 0 {
+				t.Fatal("field createdAt is empty")
+			}
+			indexData.fields[keyedField] = mutexCheckField{fieldName: fieldName, field: field, createdAt: field.CreatedAt()}
+		}
+		indexes[keyedIndex] = indexData
+	}
+
+	rowIDs := []uint64{0, 1, 2, 3}
+	colIDs := []uint64{0, 1, 2, 3}
+	rowKeysBase := []string{"v0", "v1", "v2", "v3"}
+	colKeysBase := []string{"c0", "c1", "c2", "c3"}
+
+	const nShards = 10
+
+	// now, try the same thing for each combination of keyed/unkeyed. we
+	// share code between keyed/unkeyed fields, but for indexes, the logic
+	// is fundamentally different because we can't know shards in advance.
+	indexData := indexes[false]
+	for keyedField, fieldData := range indexData.fields {
+		fmt.Println("running:", indexData.indexName, fieldData.fieldName)
+		t.Run(fmt.Sprintf("%s-%s", indexData.indexName, fieldData.fieldName), func(t *testing.T) {
+			for id, node := range nodesByID {
+				qcxsByID[id] = node.API.Txf().NewQcx()
+			}
+			for shard := uint64(0); shard < nShards; shard++ {
+				// restore row/col ID values which can get altered by imports
+				for i := range rowIDs {
+					rowIDs[i] = uint64(i)
+					colIDs[i] = (shard << shardwidth.Exponent) + uint64(i) + (shard % 4)
+				}
+				req := &pilosa.ImportRequest{
+					Index:          indexData.indexName,
+					IndexCreatedAt: indexData.createdAt,
+					Field:          fieldData.fieldName,
+					FieldCreatedAt: fieldData.createdAt,
+					Shard:          shard,
+					ColumnIDs:      colIDs,
+				}
+				if keyedField {
+					req.RowKeys = rowKeysBase
+				} else {
+					req.RowIDs = rowIDs
+				}
+				nodesForShard, err := m0.API.ShardNodes(ctx, indexData.indexName, shard)
+				if err != nil {
+					t.Fatalf("obtaining shard list: %v", err)
+				}
+				if len(nodesForShard) < 1 {
+					t.Fatalf("no nodes for shard %d", shard)
+				}
+				node := nodesByID[nodesForShard[0].ID]
+				if err := node.API.Import(ctx, qcxsByID[nodesForShard[0].ID], req); err != nil {
+					t.Fatalf("importing data: %v", err)
+				}
+			}
+			// and then we break the mutex and close the Qcxs
+			for id, node := range nodesByID {
+				field, err := node.API.Field(ctx, indexData.indexName, fieldData.fieldName)
+				if err != nil {
+					t.Fatalf("requesting field %s from node %s: %v", fieldData.fieldName, id, err)
+				}
+				pilosa.CorruptAMutex(t, field, qcxsByID[id])
+				err = qcxsByID[id].Finish()
+				if err != nil {
+					t.Fatalf("closing out transaction on node %s: %v", id, err)
+				}
+			}
+			qcx := m0.API.Txf().NewQcx()
+			defer qcx.Abort()
+
+			results, err := m0.API.MutexCheck(ctx, qcx, indexData.indexName, fieldData.fieldName)
+			if err != nil {
+				t.Fatalf("checking mutexes: %v", err)
+			}
+			// first two shards of each group of 4 should have a collision in
+			// position 1
+			expected := map[uint64]bool{
+				(0 << shardwidth.Exponent) + 1: true,
+				(1 << shardwidth.Exponent) + 1: true,
+				(4 << shardwidth.Exponent) + 1: true,
+				(5 << shardwidth.Exponent) + 1: true,
+				(8 << shardwidth.Exponent) + 1: true,
+				(9 << shardwidth.Exponent) + 1: true,
+			}
+			if keyedField {
+				mapped, ok := results.(map[uint64][]string)
+				if !ok {
+					t.Fatalf("expected map[uint64][]string, got %T", results)
+				}
+				seen := 0
+				for k, v := range mapped {
+					seen++
+					if !expected[k] {
+						t.Fatalf("expected all collisions to be 1 shards (s %% 4 in [0,1]), got %d", k)
+					}
+					if len(v) != 2 {
+						t.Fatalf("expected exactly two collisions")
+					}
+				}
+				if seen != len(expected) {
+					t.Fatalf("expected exactly %d records to have collisions", len(expected))
+				}
+			} else {
+				mapped, ok := results.(map[uint64][]uint64)
+				if !ok {
+					t.Fatalf("expected map[uint64][]uint64, got %T", results)
+				}
+				seen := 0
+				for k, v := range mapped {
+					seen++
+					if !expected[k] {
+						t.Fatalf("expected all collisions to be 1 shards (s %% 4 in [0,1]), got %d", k)
+					}
+					if len(v) != 2 {
+						t.Fatalf("expected exactly two collisions")
+					}
+				}
+				if seen != len(expected) {
+					t.Fatalf("expected exactly %d records to have collisions", len(expected))
+				}
+			}
+		})
+	}
+	indexData = indexes[true]
+	for keyedField, fieldData := range indexData.fields {
+		t.Run(fmt.Sprintf("%s-%s", indexData.indexName, fieldData.fieldName), func(t *testing.T) {
+			for id, node := range nodesByID {
+				qcxsByID[id] = node.API.Txf().NewQcx()
+			}
+			req := &pilosa.ImportRequest{
+				Index:          indexData.indexName,
+				IndexCreatedAt: indexData.createdAt,
+				Field:          fieldData.fieldName,
+				FieldCreatedAt: fieldData.createdAt,
+				Shard:          0, // ignored when using keys
+			}
+			rowKeys := make([]string, 0, len(rowKeysBase)*nShards)
+			colKeys := make([]string, 0, len(rowKeysBase)*nShards)
+			rowIDs = rowIDs[:0]
+			for shard := uint64(0); shard < nShards; shard++ {
+				for i := range rowKeysBase {
+					colKeys = append(colKeys, fmt.Sprintf("s%d-%s", shard, colKeysBase[i]))
+					if keyedField {
+						rowKeys = append(rowKeys, rowKeysBase[i])
+					} else {
+						rowIDs = append(rowIDs, uint64(i))
+					}
+				}
+			}
+			req.ColumnKeys = colKeys
+			if keyedField {
+				req.RowKeys = rowKeys
+			} else {
+				req.RowIDs = rowIDs
+			}
+			var id string
+			var node *test.Command
+			for id, node = range nodesByID {
+				break
+			}
+			if err := node.API.Import(ctx, qcxsByID[id], req); err != nil {
+				t.Fatalf("importing data: %v", err)
+			}
+			expected, err := node.API.FindIndexKeys(ctx, indexData.indexName, colKeys...)
+			if err != nil {
+				t.Fatalf("looking up index keys: %v", err)
+			}
+			for key, id := range expected {
+				// CorruptAMutex should only corrupt things in position 1 of their
+				// shards...
+				if id%(1<<shardwidth.Exponent) != 1 {
+					delete(expected, key)
+				}
+			}
+			if keyedField {
+				fieldValues, err := node.API.FindFieldKeys(ctx, indexData.indexName, fieldData.fieldName, rowKeys...)
+				if err != nil {
+					t.Fatalf("looking up field keys: %v", err)
+				}
+				// Figure out which key got the value 3, delete any records
+				// which would have had that key, because they won't be
+				// conflicts.
+				for key, value := range fieldValues {
+					if value == 3 {
+						for offset, baseKey := range rowKeysBase {
+							if baseKey == key {
+								for i := offset; i < len(rowKeys); i += len(rowKeysBase) {
+									delete(expected, colKeys[i])
+								}
+							}
+						}
+					}
+				}
+			} else {
+				// we set rowKeys to 0-1-2-... for rowKeysBase items, which
+				// tells us which keys we expect to be 3 already.
+				for i := 3; i < len(rowIDs); i += len(rowKeysBase) {
+					delete(expected, colKeys[i])
+				}
+			}
+			// and then we break the mutex and close the Qcxs
+			for id, node := range nodesByID {
+				field, err := node.API.Field(ctx, indexData.indexName, fieldData.fieldName)
+				if err != nil {
+					t.Fatalf("requesting field %s from node %s: %v", fieldData.fieldName, id, err)
+				}
+				pilosa.CorruptAMutex(t, field, qcxsByID[id])
+				err = qcxsByID[id].Finish()
+				if err != nil {
+					t.Fatalf("closing out transaction on node %s: %v", id, err)
+				}
+			}
+			qcx := m0.API.Txf().NewQcx()
+			defer qcx.Abort()
+
+			results, err := m0.API.MutexCheck(ctx, qcx, indexData.indexName, fieldData.fieldName)
+			if err != nil {
+				t.Fatalf("checking mutexes: %v", err)
+			}
+			if keyedField {
+				// this just sorta comes out this way with our hashing; these are
+				// the things which were in position 1 of their shards, and did
+				// not have a value which happens to map to 3.
+				mapped, ok := results.(map[string][]string)
+				if !ok {
+					t.Fatalf("expected map[string][]string, got %T", results)
+				}
+				seen := 0
+				for k, v := range mapped {
+					seen++
+					if _, ok := expected[k]; !ok {
+						t.Fatalf("unexpected collision on key %q", k)
+					}
+					if len(v) != 2 {
+						t.Fatalf("expected exactly two collisions")
+					}
+				}
+				if seen != len(expected) {
+					t.Fatalf("expected exactly %d records to have collisions, got %d", len(expected), seen)
+				}
+			} else {
+				mapped, ok := results.(map[string][]uint64)
+				if !ok {
+					t.Fatalf("expected map[string][]uint64, got %T", results)
+				}
+				seen := 0
+				for k, v := range mapped {
+					seen++
+					if _, ok := expected[k]; !ok {
+						t.Fatalf("unexpected collision on key %q", k)
+					}
+					if len(v) != 2 {
+						t.Fatalf("expected exactly two collisions")
+					}
+				}
+				if seen != len(expected) {
+					t.Fatalf("expected exactly %d records to have collisions, got %d", len(expected), seen)
+				}
+			}
+		})
+	}
+}
diff --git a/client.go b/client.go
index 4cd410345..d04791e60 100644
--- a/client.go
+++ b/client.go
@@ -84,6 +84,8 @@ type InternalClient interface {
 
 	GetNodeUsage(ctx context.Context, uri *URI) (map[string]NodeUsage, error)
 	GetPastQueries(ctx context.Context, uri *URI) ([]PastQueryStatus, error)
+
+	MutexCheck(ctx context.Context, uri *URI, index string, field string) (map[uint64]map[uint64][]uint64, error)
 }
 
 //===============
@@ -251,3 +253,7 @@ func (n nopInternalClient) GetNodeUsage(ctx context.Context, uri *URI) (map[stri
 func (n nopInternalClient) GetPastQueries(ctx context.Context, uri *URI) ([]PastQueryStatus, error) {
 	return nil, nil
 }
+
+func (n nopInternalClient) MutexCheck(ctx context.Context, uri *URI, index, field string) (map[uint64]map[uint64][]uint64, error) {
+	return nil, nil
+}
diff --git a/field.go b/field.go
index baec3a931..827e4484b 100644
--- a/field.go
+++ b/field.go
@@ -1229,6 +1229,23 @@ func (f *Field) Row(tx Tx, rowID uint64) (*Row, error) {
 	}
 }
 
+// mutexCheck performs a sanity-check on the available fragments for a
+// field. The return is map[column]map[shard][]values for collisions only.
+func (f *Field) MutexCheck(ctx context.Context, qcx *Qcx) (map[uint64]map[uint64][]uint64, error) {
+	if f.Type() != FieldTypeMutex {
+		return nil, errors.New("mutex check only valid for mutex fields")
+	}
+	f.mu.RLock()
+	defer f.mu.RUnlock()
+	standard := f.viewMap[viewStandard]
+	if standard == nil {
+		// no standard view present means we've never needed to create it,
+		// so it has no bits set, so it has no extra bits set.
+		return nil, nil
+	}
+	return standard.mutexCheck(ctx, qcx)
+}
+
 // SetBit sets a bit on a view within the field.
 func (f *Field) SetBit(tx Tx, rowID, colID uint64, t *time.Time) (changed bool, err error) {
 	viewName := viewStandard
diff --git a/field_internal_test.go b/field_internal_test.go
index d187d5ab9..56b90c54c 100644
--- a/field_internal_test.go
+++ b/field_internal_test.go
@@ -27,6 +27,7 @@ import (
 
 	"github.com/pilosa/pilosa/v2/pql"
 	"github.com/pilosa/pilosa/v2/roaring"
+	"github.com/pilosa/pilosa/v2/shardwidth"
 	"github.com/pilosa/pilosa/v2/testhook"
 )
 
@@ -922,3 +923,27 @@ func TestBSIGroup_TxReopenDB(t *testing.T) {
 	// the test: can we re-open a BSI fragment under Tx store
 	_ = f.Reopen()
 }
+
+func CorruptAMutex(tb testing.TB, field *Field, qcx *Qcx) {
+	v := field.view(viewStandard)
+	if v == nil {
+		tb.Fatalf("creating view failed")
+	}
+	frags := v.allFragments()
+	for _, frag := range frags {
+		func() {
+			tx, finisher, err := qcx.GetTx(Txo{Write: true, Index: field.idx, Shard: frag.shard})
+			defer finisher(&err)
+			if err != nil {
+				tb.Fatalf("getting tx: %v", err)
+			}
+			// set a bonus bit, bypassing the mutex handling
+			frag.mu.Lock()
+			_, err = frag.unprotectedSetBit(tx, 3, (frag.shard<<shardwidth.Exponent)+1)
+			frag.mu.Unlock()
+			if err != nil {
+				tb.Fatalf("setting bit: %v", err)
+			}
+		}()
+	}
+}
diff --git a/fragment.go b/fragment.go
index b3a6786be..f1b47fdc1 100644
--- a/fragment.go
+++ b/fragment.go
@@ -585,6 +585,17 @@ func (f *fragment) closeStorage() error {
 	return nil
 }
 
+// mutexCheck checks for any entries in fragment which violate the mutex
+// property of having only one value set for a given column ID.
+func (f *fragment) mutexCheck(tx Tx) (map[uint64][]uint64, error) {
+	dup := roaring.NewBitmapMutexDupFilter(f.shard << shardwidth.Exponent)
+	err := tx.ApplyFilter(f.index(), f.field(), f.view(), f.shard, 0, dup)
+	if err != nil {
+		return nil, err
+	}
+	return dup.Report(), nil
+}
+
 // row returns a row by ID.
 func (f *fragment) row(tx Tx, rowID uint64) (*Row, error) {
 	f.mu.Lock()
diff --git a/go.sum b/go.sum
index b9d5e44d7..87df86f36 100644
--- a/go.sum
+++ b/go.sum
@@ -65,7 +65,6 @@ github.com/envoyproxy/go-control-plane v0.9.0/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymF
 github.com/envoyproxy/go-control-plane v0.9.4/go.mod h1:6rpuAdCZL397s3pYoYcLgu1mIlRU8Am5FuJP05cCM98=
 github.com/envoyproxy/protoc-gen-validate v0.1.0/go.mod h1:iSmxcyjqTsJpI2R4NaDN7+kN2VEUnK/pcBlmesArF7c=
 github.com/fatih/color v1.7.0/go.mod h1:Zm6kSWBoL9eyXnKyktHP6abPY2pDugNf5KwzbycvMj4=
-github.com/fsnotify/fsnotify v1.4.7 h1:IXs+QLmnXW2CcXuY+8Mzv/fWEsPGWxqefPtCP5CnV9I=
 github.com/fsnotify/fsnotify v1.4.7/go.mod h1:jwhsz4b93w/PPRr/qN1Yymfu8t87LnFCMoQvtojpjFo=
 github.com/fsnotify/fsnotify v1.4.9 h1:hsms1Qyu0jgnwNXIxa+/V/PDsU6CfLf6CNO8H7IWoS4=
 github.com/fsnotify/fsnotify v1.4.9/go.mod h1:znqG4EE+3YCdAaPaxE2ZRY/06pZUdp0tY4IgpuI1SZQ=
@@ -95,7 +94,6 @@ github.com/golang/mock v1.3.1/go.mod h1:sBzyDLLjw3U8JLTeZvSv8jJB+tU5PVekmnlKIyFU
 github.com/golang/protobuf v1.2.0/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
 github.com/golang/protobuf v1.3.1/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
 github.com/golang/protobuf v1.3.2/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
-github.com/golang/protobuf v1.3.3 h1:gyjaxf+svBWX08ZjK86iN9geUJF0H6gp2IRKX6Nf6/I=
 github.com/golang/protobuf v1.3.3/go.mod h1:vzj43D7+SQXF/4pzW/hwtAqwc6iTitCiVSaWz5lYuqw=
 github.com/golang/protobuf v1.4.0-rc.1/go.mod h1:ceaxUfeHdC40wWswd/P6IGgMaK3YpKi5j83Wpe3EHw8=
 github.com/golang/protobuf v1.4.0-rc.1.0.20200221234624-67d41d38c208/go.mod h1:xKAWHe0F5eneWXFV3EuXVDTCmh+JuBKY0li0aMyXATA=
@@ -110,7 +108,6 @@ github.com/google/btree v1.0.0/go.mod h1:lNA+9X1NB3Zf8V7Ke586lFgjr2dZNuvo3lPJSGZ
 github.com/google/go-cmp v0.2.0/go.mod h1:oXzfMopK8JAjlY9xF4vHSVASa0yLyX7SntLO5aqRK0M=
 github.com/google/go-cmp v0.3.0/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
 github.com/google/go-cmp v0.3.1/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
-github.com/google/go-cmp v0.4.0 h1:xsAVV57WRhGj6kEIi8ReJzQlHHqcBYCElAvkovg3B/4=
 github.com/google/go-cmp v0.4.0/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-cmp v0.5.2 h1:X2ev0eStA3AbceY54o37/0PQ/UWqKEiiO2dKL5OPaFM=
 github.com/google/go-cmp v0.5.2/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
@@ -147,12 +144,10 @@ github.com/hashicorp/go-rootcerts v1.0.0/go.mod h1:K6zTfqpRlCUIjkwsN4Z+hiSfzSTQa
 github.com/hashicorp/go-sockaddr v1.0.0 h1:GeH6tui99pF4NJgfnhp+L6+FfobzVW3Ah46sLo0ICXs=
 github.com/hashicorp/go-sockaddr v1.0.0/go.mod h1:7Xibr9yA9JjQq1JpNB2Vw7kxv8xerXegt+ozgdvDeDU=
 github.com/hashicorp/go-syslog v1.0.0/go.mod h1:qPfqrKkXGihmCqbJM2mZgkZGvKG1dFdvsLplgctolz4=
-github.com/hashicorp/go-uuid v1.0.0 h1:RS8zrF7PhGwyNPOtxSClXXj9HA8feRnJzgnI1RJCSnM=
 github.com/hashicorp/go-uuid v1.0.0/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/bN7x4byOro=
 github.com/hashicorp/go-uuid v1.0.1 h1:fv1ep09latC32wFoVwnqcnKJGnMSdBanPczbHAYm1BE=
 github.com/hashicorp/go-uuid v1.0.1/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/bN7x4byOro=
 github.com/hashicorp/go.net v0.0.1/go.mod h1:hjKkEWcCURg++eb33jQU7oqQcI9XDCnUzHA0oac0k90=
-github.com/hashicorp/golang-lru v0.5.0 h1:CL2msUPvZTLb5O648aiLNJw3hnBxN2+1Jq8rCOH9wdo=
 github.com/hashicorp/golang-lru v0.5.0/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
 github.com/hashicorp/golang-lru v0.5.1 h1:0hERBMJE1eitiLkihrMvRVBYAkpHzc/J3QdDN+dAcgU=
 github.com/hashicorp/golang-lru v0.5.1/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
@@ -172,15 +167,12 @@ github.com/jtolds/gls v4.20.0+incompatible h1:xdiiI2gbIgH/gLH7ADydsJ1uDOEzR8yvV7
 github.com/jtolds/gls v4.20.0+incompatible/go.mod h1:QJZ7F/aHp+rZTRtaJ1ow/lLfFfVYBRgL+9YlvaHOwJU=
 github.com/julienschmidt/httprouter v1.2.0/go.mod h1:SYymIcj16QtmaHHD7aYtjjsJG7VTCxuUUipMqKk8s4w=
 github.com/kisielk/errcheck v1.1.0/go.mod h1:EZBBE59ingxPouuu3KfxchcWSUPOHkagtvWXihfKN4Q=
-github.com/kisielk/gotool v1.0.0 h1:AV2c/EiW3KqPNT9ZKl07ehoAGi4C5/01Cfbblndcapg=
 github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
 github.com/konsorten/go-windows-terminal-sequences v1.0.1/go.mod h1:T0+1ngSBFLxvqU3pZ+m/2kptfBszLMUkC4ZK/EgS/cQ=
 github.com/konsorten/go-windows-terminal-sequences v1.0.2/go.mod h1:T0+1ngSBFLxvqU3pZ+m/2kptfBszLMUkC4ZK/EgS/cQ=
 github.com/kr/logfmt v0.0.0-20140226030751-b84e30acd515/go.mod h1:+0opPa2QZZtGFBFZlji/RkVcI2GknAs/DXo4wKdlNEc=
-github.com/kr/pretty v0.1.0 h1:L/CwN0zerZDmRFUapSPitk6f+Q3+0za1rQkzVuMiMFI=
 github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
-github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
 github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
@@ -196,7 +188,6 @@ github.com/miekg/dns v1.0.14 h1:9jZdLNd/P4+SfEJ0TNyxYpsK8N4GtfylBLqtbYN1sbA=
 github.com/miekg/dns v1.0.14/go.mod h1:W1PPwlIAgtquWBMBEV9nkV9Cazfe8ScdGz/Lj7v3Nrg=
 github.com/mitchellh/cli v1.0.0/go.mod h1:hNIlj7HEI86fIcpObd7a0FcrxTWetlwJDGcceTlRvqc=
 github.com/mitchellh/go-homedir v1.0.0/go.mod h1:SfyaCUpYCn1Vlf4IUYiD9fPX4A5wJrkLzIz1N1q0pr0=
-github.com/mitchellh/go-homedir v1.1.0 h1:lukF9ziXFxDFPkA1vsr5zpc1XuPDn/wFntq5mG+4E0Y=
 github.com/mitchellh/go-homedir v1.1.0/go.mod h1:SfyaCUpYCn1Vlf4IUYiD9fPX4A5wJrkLzIz1N1q0pr0=
 github.com/mitchellh/go-testing-interface v1.0.0/go.mod h1:kRemZodwjscx+RGhAo8eIhFbs2+BFgRtFPeD/KE+zxI=
 github.com/mitchellh/gox v0.4.0/go.mod h1:Sd9lOJ0+aimLBi73mGofS1ycjY8lL3uZM3JPS42BGNg=
@@ -291,11 +282,9 @@ github.com/spf13/viper v1.7.0/go.mod h1:8WkrPz2fc9jxqZNCJI/76HCieCp4Q8HaLFoCha5q
 github.com/spf13/viper v1.7.1 h1:pM5oEahlgWv/WnHXpgbKz7iLIxRf65tye2Ci+XFK5sk=
 github.com/spf13/viper v1.7.1/go.mod h1:8WkrPz2fc9jxqZNCJI/76HCieCp4Q8HaLFoCha5qpdg=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
-github.com/stretchr/objx v0.1.1 h1:2vfRuCMp5sSVIDSqO8oNnWJq7mPa6KVP3iPIwFBuy8A=
 github.com/stretchr/objx v0.1.1/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
-github.com/stretchr/testify v1.4.0 h1:2E4SXV/wtOkTonXsotYi4li6zVWxYlZuYNCXe9XRJyk=
 github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81PSLYec5m4=
 github.com/stretchr/testify v1.6.1 h1:hDPOHmpOpP40lSULcqw7IrRb/u7w6RpDC9399XyoNd0=
 github.com/stretchr/testify v1.6.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
@@ -401,7 +390,6 @@ golang.org/x/sys v0.0.0-20191001151750-bb3f8db39f24/go.mod h1:h1NjWce9XRLGQEsW7w
 golang.org/x/sys v0.0.0-20191005200804-aed5e4c7ecf9/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20191220142924-d4481acd189f/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20200202164722-d101bd2416d5/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20200323222414-85ca7c5b95cd h1:xhmwyvizuTgC2qz7ZlMluP20uW+C3Rm0FD/WLDX8884=
 golang.org/x/sys v0.0.0-20200323222414-85ca7c5b95cd/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20201024232916-9f70ab9862d5/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20201214095126-aec9a390925b h1:tv7/y4pd+sR8bcNb2D6o7BNU6zjWm0VjQLac+w7fNNM=
@@ -453,7 +441,6 @@ google.golang.org/genproto v0.0.0-20190418145605-e7d98fc518a7/go.mod h1:VzzqZJRn
 google.golang.org/genproto v0.0.0-20190425155659-357c62f0e4bb/go.mod h1:VzzqZJRnGkLBvHegQrXjBqPurQTc5/KpmUdxsrq26oE=
 google.golang.org/genproto v0.0.0-20190502173448-54afdca5d873/go.mod h1:VzzqZJRnGkLBvHegQrXjBqPurQTc5/KpmUdxsrq26oE=
 google.golang.org/genproto v0.0.0-20190801165951-fa694d86fc64/go.mod h1:DMBHOl98Agz4BDEuKkezgsaosCRResVns1a3J2ZsMNc=
-google.golang.org/genproto v0.0.0-20190819201941-24fa4b261c55 h1:gSJIx1SDwno+2ElGhA4+qG2zF97qiUzTM+rQ0klBOcE=
 google.golang.org/genproto v0.0.0-20190819201941-24fa4b261c55/go.mod h1:DMBHOl98Agz4BDEuKkezgsaosCRResVns1a3J2ZsMNc=
 google.golang.org/genproto v0.0.0-20190911173649-1774047e7e51/go.mod h1:IbNlFCBrqXvoKpeg0TB2l7cyZUmoaFKYIwrEpbDKLA8=
 google.golang.org/genproto v0.0.0-20191108220845-16a3f7862a1a h1:Ob5/580gVHBJZgXnff1cZDbG+xLtMVE5mDRTe+nIsX4=
@@ -483,7 +470,6 @@ gopkg.in/ini.v1 v1.51.0/go.mod h1:pNLf8WUiyNEtQjuu5G5vTm06TEv9tsIgeAvK8hOrP4k=
 gopkg.in/resty.v1 v1.12.0/go.mod h1:mDo4pnntr5jdWRML875a/NmxYqAlA73dVijT2AXvQQo=
 gopkg.in/yaml.v2 v2.0.0-20170812160011-eb3733d160e7/go.mod h1:JAlM8MvJe8wmxCU4Bli9HhUf9+ttbYbLASfIpnQbh74=
 gopkg.in/yaml.v2 v2.2.1/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.4/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.8/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
diff --git a/http/client.go b/http/client.go
index 4f37d7a36..ce8d70e13 100644
--- a/http/client.go
+++ b/http/client.go
@@ -133,6 +133,34 @@ func (c *InternalClient) Schema(ctx context.Context) ([]*pilosa.IndexInfo, error
 	return rsp.Indexes, nil
 }
 
+// MutexCheck uses the mutex-check endpoint to request mutex collision data
+// from a single node.
+func (c *InternalClient) MutexCheck(ctx context.Context, uri *pilosa.URI, indexName string, fieldName string) (map[uint64]map[uint64][]uint64, error) {
+	if uri == nil {
+		uri = c.defaultURI
+	}
+	u := uri.Path(fmt.Sprintf("/internal/index/%s/field/%s/mutex-check", indexName, fieldName))
+	req, err := http.NewRequest("GET", u, nil)
+	if err != nil {
+		return nil, errors.Wrap(err, "creating request")
+	}
+	req.Header.Set("Accept", "application/json")
+	req.Header.Set("User-Agent", "pilosa/"+pilosa.Version)
+
+	resp, err := c.executeRequest(req.WithContext(ctx))
+	if err != nil {
+		return nil, errors.Wrap(err, "executing request")
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return nil, errors.Errorf("unexpected status code: %s", resp.Status)
+	}
+	var out map[uint64]map[uint64][]uint64
+	dec := json.NewDecoder(resp.Body)
+	err = dec.Decode(&out)
+	return out, err
+}
+
 func (c *InternalClient) PostSchema(ctx context.Context, uri *pilosa.URI, s *pilosa.Schema, remote bool) error {
 	u := uri.Path(fmt.Sprintf("/schema?remote=%v", remote))
 	buf, err := json.Marshal(s)
diff --git a/http/handler.go b/http/handler.go
index a6dfd5828..2476afd89 100644
--- a/http/handler.go
+++ b/http/handler.go
@@ -386,6 +386,7 @@ func newRouter(handler *Handler) http.Handler {
 	router.HandleFunc("/index/{index}/field/{field}", handler.handlePostField).Methods("POST").Name("PostField")
 	router.HandleFunc("/index/{index}/field/{field}", handler.handleDeleteField).Methods("DELETE").Name("DeleteField")
 	router.HandleFunc("/index/{index}/field/{field}/import", handler.handlePostImport).Methods("POST").Name("PostImport")
+	router.HandleFunc("/index/{index}/field/{field}/mutex-check", handler.handleGetMutexCheck).Methods("GET").Name("GetMutexCheck")
 	router.HandleFunc("/index/{index}/field/{field}/import-roaring/{shard}", handler.handlePostImportRoaring).Methods("POST").Name("PostImportRoaring")
 	router.HandleFunc("/index/{index}/query", handler.handlePostQuery).Methods("POST").Name("PostQuery")
 	router.HandleFunc("/info", handler.handleGetInfo).Methods("GET").Name("GetInfo")
@@ -422,6 +423,7 @@ func newRouter(handler *Handler) http.Handler {
 	router.HandleFunc("/internal/translate/data", handler.handlePostTranslateData).Methods("POST").Name("PostTranslateData")
 	router.HandleFunc("/internal/translate/keys", handler.handlePostTranslateKeys).Methods("POST").Name("PostTranslateKeys")
 	router.HandleFunc("/internal/translate/ids", handler.handlePostTranslateIDs).Methods("POST").Name("PostTranslateIDs")
+	router.HandleFunc("/internal/index/{index}/field/{field}/mutex-check", handler.handleInternalGetMutexCheck).Methods("GET").Name("InternalGetMutexCheck")
 	router.HandleFunc("/internal/index/{index}/field/{field}/attr/diff", handler.handlePostFieldAttrDiff).Methods("POST").Name("PostFieldAttrDiff")
 	router.HandleFunc("/internal/index/{index}/field/{field}/remote-available-shards/{shardID}", handler.handleDeleteRemoteAvailableShard).Methods("DELETE")
 	router.HandleFunc("/internal/nodes", handler.handleGetNodes).Methods("GET").Name("GetNodes")
@@ -2479,6 +2481,58 @@ func (h *Handler) handlePostImportColumnAttrs(w http.ResponseWriter, r *http.Req
 	}
 }
 
+// handleGetMutexCheck handles /mutex-check requests.
+func (h *Handler) handleGetMutexCheck(w http.ResponseWriter, r *http.Request) {
+	if !validHeaderAcceptJSON(r.Header) {
+		http.Error(w, "JSON only acceptable response", http.StatusNotAcceptable)
+		return
+	}
+	// Get index and field type to determine how to handle the
+	// import data.
+	indexName, fieldName := mux.Vars(r)["index"], mux.Vars(r)["field"]
+	qcx := h.api.Txf().NewQcx()
+	defer qcx.Abort()
+	out, err := h.api.MutexCheck(r.Context(), qcx, indexName, fieldName)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	outBytes, err := json.Marshal(out)
+	if err != nil {
+		http.Error(w, fmt.Sprintf("marshalling response: %v", err), http.StatusInternalServerError)
+	}
+	_, err = w.Write(outBytes)
+	if err != nil {
+		h.logger.Printf("writing mutex-check response: %v", err)
+	}
+}
+
+// handleInternalGetMutexCheck handles internal (non-forwarding )/mutex-check requests.
+func (h *Handler) handleInternalGetMutexCheck(w http.ResponseWriter, r *http.Request) {
+	if !validHeaderAcceptJSON(r.Header) {
+		http.Error(w, "JSON only acceptable response", http.StatusNotAcceptable)
+		return
+	}
+	// Get index and field type to determine how to handle the
+	// import data.
+	indexName, fieldName := mux.Vars(r)["index"], mux.Vars(r)["field"]
+	qcx := h.api.Txf().NewQcx()
+	defer qcx.Abort()
+	out, err := h.api.MutexCheckNode(r.Context(), qcx, indexName, fieldName)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	outBytes, err := json.Marshal(out)
+	if err != nil {
+		http.Error(w, fmt.Sprintf("marshalling response: %v", err), http.StatusInternalServerError)
+	}
+	_, err = w.Write(outBytes)
+	if err != nil {
+		h.logger.Printf("writing mutex-check response: %v", err)
+	}
+}
+
 // handlePostImportRoaring
 func (h *Handler) handlePostImportRoaring(w http.ResponseWriter, r *http.Request) {
 	// Verify that request is only communicating over protobufs.
diff --git a/roaring/filter.go b/roaring/filter.go
index 1f2018ad1..9cdb7f24d 100644
--- a/roaring/filter.go
+++ b/roaring/filter.go
@@ -755,6 +755,75 @@ func NewBitmapRangeFilter(min, max FilterKey, keyCallback func(FilterKey, int32)
 	return &BitmapRangeFilter{min: min, max: max, kcb: keyCallback, dcb: dataCallback}
 }
 
+// BitmapMutexDupFilter is a filter which identifies cases where the same
+// position has a bit set in more than one row.
+//
+// We keep a slice of the first value seen for every row, with ^0 as the
+// default; when that's already set, things get appended to the entries in
+// the map. At the end, for each entry in the map, we also add its first
+// value to it. Thus, the map holds all the entries, but we're only using
+// the map in the (hopefully rarer) cases where there's duplicate values.
+//
+// The slice is local-coordinates (first column 0), but the map is global
+// coordinates (first column is whatever base was).
+type BitmapMutexDupFilter struct {
+	base  uint64              // the offset of 0 for this, used to accommodate shard offsets
+	extra map[uint64][]uint64 // extra values observed
+	first []uint64            // first values observed
+}
+
+var _ BitmapFilter = &BitmapMutexDupFilter{}
+
+func NewBitmapMutexDupFilter(base uint64) *BitmapMutexDupFilter {
+	filter := &BitmapMutexDupFilter{
+		base:  base,
+		extra: map[uint64][]uint64{},
+		first: make([]uint64, 1<<shardwidth.Exponent),
+	}
+	for i := range filter.first {
+		filter.first[i] = ^uint64(0)
+	}
+	return filter
+}
+
+func (b *BitmapMutexDupFilter) ConsiderKey(key FilterKey, n int32) FilterResult {
+	if n > 0 {
+		return key.NeedData()
+	}
+	return key.RejectOne()
+}
+
+func (b *BitmapMutexDupFilter) ConsiderData(key FilterKey, data *Container) FilterResult {
+	value, basePos := uint64(key)>>rowExponent, uint64(key&keyMask)<<16
+	containerCallback(data, func(u uint16) {
+		pos := basePos + uint64(u)
+		if b.first[pos] != ^uint64(0) {
+			b.extra[pos+b.base] = append(b.extra[pos+b.base], value)
+		} else {
+			b.first[pos] = value
+		}
+	})
+	return key.MatchOne()
+}
+
+// Report returns the set of duplicate values identified.
+func (b *BitmapMutexDupFilter) Report() map[uint64][]uint64 {
+	// copy values into extra, and remove them from first, so calling
+	// Report() again won't cause double-appends.
+	for k, v := range b.extra {
+		kpos := k % (1 << shardwidth.Exponent)
+		if b.first[kpos] != ^uint64(0) {
+			v = append(v, 0)
+			// prepend so the lowest value goes at the beginning
+			copy(v[1:], v[:])
+			v[0] = b.first[kpos]
+			b.first[kpos] = ^uint64(0)
+			b.extra[k] = v
+		}
+	}
+	return b.extra
+}
+
 // ApplyFilterToIterator is a simplistic implementation that applies a bitmap
 // filter to a ContainerIterator, returning an error if it encounters an error.
 //
diff --git a/roaring/filter_internal_test.go b/roaring/filter_internal_test.go
index 0fc9c0fe0..1811acff4 100644
--- a/roaring/filter_internal_test.go
+++ b/roaring/filter_internal_test.go
@@ -349,3 +349,50 @@ func TestFilterWithRows(t *testing.T) {
 	}
 
 }
+
+func TestMutexDupFilter(t *testing.T) {
+	tests := []struct{
+		pairs [][2]uint64
+		expect map[uint64][]uint64
+	}{
+		{
+			pairs: [][2]uint64{{0, 0}, {1, 0}, {0, 1}},
+			expect: map[uint64][]uint64{0: {0, 1}},
+		},
+		{
+			pairs: [][2]uint64{{0, 0}, {1, 0}, {0, 1}, {0, 2}},
+			expect: map[uint64][]uint64{0: {0, 1, 2}},
+		},
+	}
+	for num, test := range tests {
+		t.Run(fmt.Sprintf("case%d", num), func(t *testing.T) {
+			b := NewSliceBitmap()
+			for _, p := range test.pairs {
+				v := (p[1] << shardwidth.Exponent) | p[0]
+				b.DirectAdd(v)
+			}
+			dup := NewBitmapMutexDupFilter(0)
+			iter, _ := b.Containers.Iterator(0)
+			err := ApplyFilterToIterator(dup, iter)
+			if err != nil {
+				t.Fatalf("applying filter: %v", err)
+			}
+			expected := test.expect
+			got := dup.Report()
+			if len(expected) != len(got) {
+				t.Fatalf("expected %d entries in duplicate map, got %d", len(expected), len(got))
+			}
+			for k, v := range expected {
+				gv := got[k]
+				if len(v) != len(gv) {
+					t.Fatalf("for id %d, expected %d (len %d), got %d (len %d)", k, v, len(v), gv, len(gv))
+				}
+				for j := range v {
+					if gv[j] != v[j] {
+						t.Fatalf("for id %d, expected %d, got %d", k, v[j], gv[j])
+					}
+				}
+			}
+		})
+	}
+}
diff --git a/roaring/roaring.go b/roaring/roaring.go
index 5504b9bf1..32d573911 100644
--- a/roaring/roaring.go
+++ b/roaring/roaring.go
@@ -4448,10 +4448,13 @@ func intersectionCallbackArrayArray(a, b *Container, fn func(uint16)) {
 	if (na << 2) < nb {
 		for _, va := range ca {
 			for cb[0] < va {
-				if len(cb) > 8 && cb[0] < va {
+				// try to skip ahead a bit faster
+				for len(cb) > 7 && cb[7] < va {
 					cb = cb[8:]
 				}
-				cb = cb[1:]
+				for len(cb) > 0 && cb[0] < va {
+					cb = cb[1:]
+				}
 				if len(cb) == 0 {
 					return
 				}
@@ -4540,7 +4543,7 @@ func intersectionCallbackBitmapRun(a, b *Container, fn func(uint16)) {
 	}
 }
 
-func intersectionCallbackArrayBitmap(a, b *Container, fn func(uint16)) (n int32) {
+func intersectionCallbackArrayBitmap(a, b *Container, fn func(uint16)) {
 	statsHit("intersectionCount/ArrayBitmap")
 	bitmap := b.bitmap()
 	ln := len(bitmap)
@@ -4550,9 +4553,10 @@ func intersectionCallbackArrayBitmap(a, b *Container, fn func(uint16)) (n int32)
 			break
 		}
 		off := val % 64
-		n += int32(bitmap[i]>>off) & 1
+		if (bitmap[i]>>off)&1 != 0 {
+			fn(val)
+		}
 	}
-	return n
 }
 
 func intersectionCallbackBitmapBitmap(a, b *Container, fn func(uint16)) {
diff --git a/view.go b/view.go
index 15173f5e4..0af36b71a 100644
--- a/view.go
+++ b/view.go
@@ -447,6 +447,50 @@ func (v *view) row(txOrig Tx, rowID uint64) (*Row, error) {
 
 }
 
+// mutexCheck checks all available fragments for duplicate values. The return
+// is map[column]map[shard][]values for collisions only.
+func (v *view) mutexCheck(ctx context.Context, qcx *Qcx) (map[uint64]map[uint64][]uint64, error) {
+	// We don't need the context, we just want the context-awareness on the error groups.
+	// It would be nice if the inner functions could use this too...
+	eg, _ := errgroup.WithContext(ctx)
+	throttle := make(chan struct{}, runtime.NumCPU())
+	frags := v.allFragments()
+	results := make([]map[uint64][]uint64, len(frags))
+	for i, frag := range frags {
+		// local copies for the goroutine to use
+		i, frag := i, frag
+		eg.Go(func() error {
+			// limit simultaneous parallel goroutines associated with this
+			throttle <- struct{}{}
+			defer func() {
+				<-throttle
+			}()
+			tx, finisher, err := qcx.GetTx(Txo{Index: v.idx, Shard: frag.shard})
+			if err != nil {
+				return err
+			}
+			defer finisher(&err)
+			results[i], err = frag.mutexCheck(tx)
+			if err != nil {
+				return err
+			}
+			return nil
+		})
+	}
+	err := eg.Wait()
+	if err != nil {
+		return nil, err
+	}
+	out := map[uint64]map[uint64][]uint64{}
+	for i, result := range results {
+		if len(result) == 0 {
+			continue
+		}
+		out[frags[i].shard] = result
+	}
+	return out, nil
+}
+
 // setBit sets a bit within the view.
 func (v *view) setBit(txOrig Tx, rowID, columnID uint64) (changed bool, err error) {
 	shard := columnID / ShardWidth