diff --git a/pql/ast.go b/pql/ast.go
index d78ad2828..36d6e35fb 100644
--- a/pql/ast.go
+++ b/pql/ast.go
@@ -103,7 +103,9 @@ func (q *Query) endConditional() {
 
 func (q *Query) addField(field string) {
 	elem := q.lastCallStackElem()
-	if elem == nil || elem.lastField != "" {
+	if elem == nil {
+		panic(fmt.Sprintf("addField called with '%s' while element is nil", field))
+	} else if elem.lastField != "" {
 		panic(fmt.Sprintf("addField called with '%s' while field is not empty, it's: %s", field, elem.lastField))
 	}
 	elem.lastField = field
@@ -112,6 +114,15 @@ func (q *Query) addField(field string) {
 	}
 }
 
+// validateArgField ensures that field does not already
+// exist as a key in the Args map before adding the new
+// key/value.
+func (q *Query) validateArgField(elem *callStackElem) {
+	if _, exists := elem.call.Args[elem.lastField]; exists {
+		panic(fmt.Sprintf("multiple instances of argument '%s' provided", elem.lastField))
+	}
+}
+
 func (q *Query) addVal(val interface{}) {
 	elem := q.lastCallStackElem()
 	if elem == nil || elem.lastField == "" {
@@ -123,11 +134,13 @@ func (q *Query) addVal(val interface{}) {
 		return
 	}
 	if elem.lastCond != ILLEGAL {
+		q.validateArgField(elem) // case 1
 		elem.call.Args[elem.lastField] = &Condition{
 			Op:    elem.lastCond,
 			Value: val,
 		}
 	} else {
+		q.validateArgField(elem) // case 2
 		elem.call.Args[elem.lastField] = val
 	}
 	elem.lastField = ""
@@ -162,11 +175,13 @@ func (q *Query) addNumVal(val string) {
 		}
 		return
 	} else if elem.lastCond != ILLEGAL {
+		q.validateArgField(elem) // case 3
 		elem.call.Args[elem.lastField] = &Condition{
 			Op:    elem.lastCond,
 			Value: ival,
 		}
 	} else {
+		q.validateArgField(elem) // case 4
 		elem.call.Args[elem.lastField] = ival
 	}
 	elem.lastField = ""
@@ -175,6 +190,7 @@ func (q *Query) addNumVal(val string) {
 
 func (q *Query) startList() {
 	elem := q.lastCallStackElem()
+	q.validateArgField(elem) // case 5
 	if elem.lastCond != ILLEGAL {
 		elem.call.Args[elem.lastField] = &Condition{
 			Op:    elem.lastCond,
diff --git a/pql/pqlpeg_test.go b/pql/pqlpeg_test.go
index 8492d3531..f8a6597b5 100644
--- a/pql/pqlpeg_test.go
+++ b/pql/pqlpeg_test.go
@@ -676,3 +676,55 @@ func TestPQLDeepEquality(t *testing.T) {
 		})
 	}
 }
+
+func TestPQLPanic(t *testing.T) {
+	tests := []struct {
+		name string
+		call string
+	}{
+		// case 1
+		{
+			name: "StringConditional",
+			call: "Row(a==foo, a==bar)",
+		},
+		// case 2
+		{
+			name: "StringValue",
+			call: "Row(a=foo, a=bar)",
+		},
+		// case 3
+		{
+			name: "IntConditional",
+			call: "Row(a>5, a>6)",
+		},
+		// case 4
+		{
+			name: "IntValue",
+			call: "Row(a=7, a=8)",
+		},
+		// case 5
+		{
+			name: "List",
+			call: "Row(a=[7], a=[7,8])",
+		},
+	}
+	for i, test := range tests {
+		t.Run(test.name+strconv.Itoa(i), func(t *testing.T) {
+			var v interface{}
+			func() {
+				defer func() { v = recover() }()
+
+				q, err := ParseString(test.call)
+				if err != nil {
+					t.Fatalf("parsing query '%s': %v", test.call, err)
+				}
+				_ = q
+			}()
+
+			if !reflect.DeepEqual(v, "multiple instances of argument 'a' provided") {
+				t.Fatalf("unexpected panic value: %#v", v)
+			}
+
+		})
+	}
+}