diff --git a/chrome/content/zotero/xpcom/data/item.js b/chrome/content/zotero/xpcom/data/item.js
index 451018d6f2..346ece00cd 100644
--- a/chrome/content/zotero/xpcom/data/item.js
+++ b/chrome/content/zotero/xpcom/data/item.js
@@ -4208,11 +4208,11 @@ Zotero.Item.prototype.fromJSON = function (json, options = {}) {
 	
 	var isValidForType = {};
 	var setFields = new Set();
-	/*var { fields: extraFields, creators: extraCreators, extra } = Zotero.Utilities.Internal.extractExtraFields(
+	var { fields: extraFields, creators: extraCreators, extra } = Zotero.Utilities.Internal.extractExtraFields(
 		json.extra !== undefined ? json.extra : '',
 		this,
 		Object.keys(json)
-	);*/
+	);
 	
 	// Transfer valid fields from Extra to regular fields
 	// Currently disabled
@@ -4319,12 +4319,11 @@ Zotero.Item.prototype.fromJSON = function (json, options = {}) {
 					throw e;
 				}
 				// Otherwise store in Extra
-				// TEMP: Disabled for now, along with tests in itemTest.js
-				/*if (typeof val == 'string') {
+				if (typeof val == 'string') {
 					Zotero.warn(`Storing unknown field '${field}' in Extra for item ${this.libraryKey}`);
 					extraFields.set(field, val);
 					break;
-				}*/
+				}
 				Zotero.warn(`Discarding unknown JSON ${typeof val} '${field}' for item ${this.libraryKey}`);
 				continue;
 			}
@@ -4346,12 +4345,9 @@ Zotero.Item.prototype.fromJSON = function (json, options = {}) {
 					throw e;
 				}
 				// Otherwise store in Extra
-				// TEMP: Disabled for now, since imports can assign values to multiple versions of
-				// fields
-				// https://groups.google.com/d/msg/zotero-dev/a1IPUJ2m_3s/hfmdK2P3BwAJ
-				/*Zotero.warn(`Storing invalid field '${origField}' for type ${type} in Extra for `
+				Zotero.warn(`Storing invalid field '${origField}' for type ${type} in Extra for `
 					+ `item ${this.libraryKey}`);
-				extraFields.set(field, val);*/
+				extraFields.set(field, val);
 				continue;
 			}
 			this.setField(field, json[origField]);
@@ -4359,8 +4355,37 @@ Zotero.Item.prototype.fromJSON = function (json, options = {}) {
 		}
 	}
 	
-	//this.setField('extra', Zotero.Utilities.Internal.combineExtraFields(extra, extraFields));
-	this.setField('extra', json.extra !== undefined ? json.extra : '');
+	// If one of the valid fields is a base field or a base-mapped field, remove all other
+	// associated fields from Extra. This could be removed if we made sure that translators didn't
+	// try to save multiple versions of base-mapped fields, which they shouldn't need to do.
+	//
+	// https://github.com/zotero/zotero/issues/1504#issuecomment-572415083
+	if (extraFields.size) {
+		for (let field of setFields.keys()) {
+			let baseField;
+			if (Zotero.ItemFields.isBaseField(field)) {
+				baseField = field;
+			}
+			else {
+				let baseFieldID = Zotero.ItemFields.getBaseIDFromTypeAndField(itemTypeID, field);
+				if (baseFieldID) {
+					baseField = baseFieldID;
+				}
+			}
+			if (baseField) {
+				let mappedFieldNames = Zotero.ItemFields.getTypeFieldsFromBase(baseField, true);
+				for (let mappedField of mappedFieldNames) {
+					if (extraFields.has(mappedField)) {
+						Zotero.warn(`Removing redundant Extra field '${mappedField}' for item `
+							+ this.libraryKey);
+						extraFields.delete(mappedField);
+					}
+				}
+			}
+		}
+	}
+	
+	this.setField('extra', Zotero.Utilities.Internal.combineExtraFields(extra, extraFields));
 	
 	if (json.collections || this._collections.length) {
 		this.setCollections(json.collections);
diff --git a/chrome/content/zotero/xpcom/utilities_internal.js b/chrome/content/zotero/xpcom/utilities_internal.js
index f2a6439e08..f6eb79a2d3 100644
--- a/chrome/content/zotero/xpcom/utilities_internal.js
+++ b/chrome/content/zotero/xpcom/utilities_internal.js
@@ -1131,7 +1131,7 @@ Zotero.Utilities.Internal = {
 			}
 		}
 		var fieldPairs = Array.from(fields.entries())
-			.map(x => x[0] + ': ' + x[1]);
+			.map(x => this.camelToTitleCase(x[0]) + ': ' + x[1]);
 		fieldPairs.sort();
 		return fieldPairs.join('\n')
 			+ ((fieldPairs.length && keepLines.length) ? "\n" : "")
@@ -1382,7 +1382,7 @@ Zotero.Utilities.Internal = {
 	
 	
 	camelToTitleCase: function (str) {
-		str = str.replace(/([A-Z])/g, " $1");
+		str = str.replace(/([a-z])([A-Z])/g, "$1 $2");
 		return str.charAt(0).toUpperCase() + str.slice(1);
 	},
 	
diff --git a/test/tests/itemTest.js b/test/tests/itemTest.js
index 3df03e0430..2d22dcb7f4 100644
--- a/test/tests/itemTest.js
+++ b/test/tests/itemTest.js
@@ -1754,6 +1754,18 @@ describe("Zotero.Item", function () {
 			assert.isFalse(item.inPublications);
 		});
 		
+		it("should handle Extra in non-strict mode", function () {
+			var json = {
+				itemType: "journalArticle",
+				title: "Test",
+				extra: "Here's some extra text"
+			};
+			var item = new Zotero.Item();
+			item.fromJSON(json);
+			assert.equal(item.getField('extra'), json.extra);
+		});
+		
+		
 		// Not currently following this behavior
 		/*it("should move valid field in Extra to field if not set", function () {
 			var doi = '10.1234/abcd';
@@ -1799,19 +1811,20 @@ describe("Zotero.Item", function () {
 			assert.equal(item.getField('extra'), `doi: ${doi2}`);
 		});*/
 		
-		it.skip("should store unknown field in Extra in non-strict mode", function () {
+		it("should store unknown fields in Extra in non-strict mode", function () {
 			var json = {
 				itemType: "journalArticle",
 				title: "Test",
-				foo: "Bar"
+				fooBar: "123",
+				testField: "test value"
 			};
 			var item = new Zotero.Item;
 			item.fromJSON(json);
 			assert.equal(item.getField('title'), 'Test');
-			assert.equal(item.getField('extra'), 'foo: Bar');
+			assert.equal(item.getField('extra'), 'Foo Bar: 123\nTest Field: test value');
 		});
 		
-		it.skip("should replace unknown field in Extra in non-strict mode", function () {
+		it("should replace unknown field in Extra in non-strict mode", function () {
 			var json = {
 				itemType: "journalArticle",
 				title: "Test",
@@ -1824,15 +1837,42 @@ describe("Zotero.Item", function () {
 			assert.equal(item.getField('extra'), 'Foo: BBB\nBar: CCC');
 		});
 		
-		it("should handle Extra in non-strict mode", function () {
+		it("should store invalid-for-type field in Extra in non-strict mode", function () {
 			var json = {
 				itemType: "journalArticle",
 				title: "Test",
-				extra: "Here's some extra text"
+				medium: "123"
 			};
-			var item = new Zotero.Item();
+			var item = new Zotero.Item;
 			item.fromJSON(json);
-			assert.equal(item.getField('extra'), json.extra);
+			assert.equal(item.getField('title'), 'Test');
+			assert.equal(item.getField('extra'), 'Medium: 123');
+		});
+		
+		it("should ignore invalid-for-type base-mapped field if valid-for-type base field is set in Extra in non-strict mode", function () {
+			var json = {
+				itemType: "document",
+				publisher: "Foo", // Valid for 'document'
+				company: "Bar" // Not invalid for 'document', but mapped to base field 'publisher'
+			};
+			var item = new Zotero.Item;
+			item.fromJSON(json);
+			assert.equal(item.getField('publisher'), 'Foo');
+			assert.equal(item.getField('extra'), '');
+		});
+		
+		it("shouldn't include base field or invalid base-mapped field in Extra if valid base-mapped field is set in non-strict mode", function () {
+			var json = {
+				itemType: "audioRecording",
+				publisher: "A", // Base field, which will be overwritten by the valid base-mapped field
+				label: "B", // Valid base-mapped field, which should be stored
+				company: "C", // Invalid base-mapped field, which should be ignored
+				foo: "D" // Invalid other field, which should be added to Extra
+			};
+			var item = new Zotero.Item;
+			item.fromJSON(json);
+			assert.equal(item.getField('label'), 'B');
+			assert.equal(item.getField('extra'), 'Foo: D');
 		});
 		
 		it("should throw on unknown field in strict mode", function () {
diff --git a/test/tests/utilities_internalTest.js b/test/tests/utilities_internalTest.js
index 73191131d1..b4db6f431b 100644
--- a/test/tests/utilities_internalTest.js
+++ b/test/tests/utilities_internalTest.js
@@ -253,7 +253,7 @@ describe("Zotero.Utilities.Internal", function () {
 		fieldMap.set('originalDate', originalDate);
 		fieldMap.set('publicationPlace', publicationPlace);
 		fieldMap.set('DOI', doi);
-		var fieldStr = `DOI: ${doi}\noriginalDate: ${originalDate}\npublicationPlace: ${publicationPlace}`;
+		var fieldStr = `DOI: ${doi}\nOriginal Date: ${originalDate}\nPublication Place: ${publicationPlace}`;
 		
 		it("should create 'field: value' pairs from field map", function () {
 			var extra = "";
@@ -272,7 +272,7 @@ describe("Zotero.Utilities.Internal", function () {
 			var newExtra = ZUI.combineExtraFields(extra, fieldMap);
 			assert.equal(
 				newExtra,
-				fieldStr.split(/\n/).filter(x => !x.startsWith('originalDate')).join("\n")
+				fieldStr.split(/\n/).filter(x => !x.startsWith('Original Date')).join("\n")
 					+ "\nThis is a note.\nOriginal Date: 1887\nFoo: Bar"
 			);
 		});
@@ -386,6 +386,21 @@ describe("Zotero.Utilities.Internal", function () {
 		});
 	});
 	
+	describe("#camelToTitleCase()", function () {
+		it("should convert 'fooBar' to 'Foo Bar'", function () {
+			assert.equal(Zotero.Utilities.Internal.camelToTitleCase('fooBar'), 'Foo Bar');
+		});
+		
+		it("should keep all-caps strings intact", function () {
+			assert.equal(Zotero.Utilities.Internal.camelToTitleCase('DOI'), 'DOI');
+		});
+		
+		it("should convert 'fooBAR' to 'Foo BAR'", function () {
+			assert.equal(Zotero.Utilities.Internal.camelToTitleCase('fooBAR'), 'Foo BAR');
+		});
+
+	});
+	
 	describe("#getNextName()", function () {
 		it("should get the next available numbered name", function () {
 			var existing = ['Name', 'Name 1', 'Name 3'];