diff --git a/src/vs/workbench/services/preferences/common/preferencesModels.ts b/src/vs/workbench/services/preferences/common/preferencesModels.ts
index 2f334c3ca5a..e0f621a144a 100644
--- a/src/vs/workbench/services/preferences/common/preferencesModels.ts
+++ b/src/vs/workbench/services/preferences/common/preferencesModels.ts
@@ -1043,6 +1043,13 @@ export function createValidator(prop: IConfigurationPropertySchema): (value: any
 
 				const stringArrayValue = value as string[];
 
+				if (prop.uniqueItems) {
+					if (new Set(stringArrayValue).size < stringArrayValue.length) {
+						message += nls.localize('validations.stringArrayUniqueItems', 'Array has duplicate items');
+						message += '\n';
+					}
+				}
+
 				if (prop.minItems && stringArrayValue.length < prop.minItems) {
 					message += nls.localize('validations.stringArrayMinItem', 'Array must have at least {0} items', prop.minItems);
 					message += '\n';
diff --git a/src/vs/workbench/services/preferences/test/common/preferencesModel.test.ts b/src/vs/workbench/services/preferences/test/common/preferencesModel.test.ts
index b02e29c0c96..9fbfb467a2b 100644
--- a/src/vs/workbench/services/preferences/test/common/preferencesModel.test.ts
+++ b/src/vs/workbench/services/preferences/test/common/preferencesModel.test.ts
@@ -328,4 +328,10 @@ suite('Preferences Model test', () => {
 
 		arr.rejects(['a']).withMessage(`err: must be friendly`);
 	});
+
+	test('uniqueItems', () => {
+		const arr = new ArrayTester({ type: 'array', items: { type: 'string' }, uniqueItems: true });
+
+		arr.rejects(['a', 'a']).withMessage(`Array has duplicate items`);
+	});
 });