diff --git a/backend/src/v5/models/users.js b/backend/src/v5/models/users.js
index c331ac7508..5f2faa93e2 100644
--- a/backend/src/v5/models/users.js
+++ b/backend/src/v5/models/users.js
@@ -164,7 +164,7 @@ User.removeUsers = (users) => db.deleteMany(USERS_DB_NAME, USERS_COL, { user: {
 
 User.ensureIndicesExist = async () => {
 	try {
-		await db.createIndex(USERS_DB_NAME, USERS_COL, { 'customData.userId': 1 }, { runInBackground: true, unique: true });
+		await db.createIndex(USERS_DB_NAME, USERS_COL, { 'customData.userId': 1 }, { runInBackground: true });
 	} catch (err) {
 		// Note this will fail pre 5.16 migration.
 		logger.logWarning('Failed to create index on user ID. Please ensure 5.16 migration script has been executed.');
diff --git a/backend/tests/v5/drivers/handler/db.test.js b/backend/tests/v5/drivers/handler/db.test.js
index 4306fef824..f495f83d61 100644
--- a/backend/tests/v5/drivers/handler/db.test.js
+++ b/backend/tests/v5/drivers/handler/db.test.js
@@ -765,6 +765,22 @@ const testIndices = () => {
 			const array = res.map(({ key }) => key);
 			expect(array).toEqual(expect.arrayContaining([{ _id: 1 }, ...newIndex.map(({ key }) => key)]));
 		});
+
+		test('Should be able to create unique index', async () => {
+			const database = generateRandomString();
+			const col = generateRandomString();
+
+			await DB.insertMany(database, col, [{ n: 2 }, { a: 1 }]);
+
+			const newIndex = { n: 1 };
+			await expect(DB.createIndex(database, col, newIndex, { unique: true })).resolves.toBeUndefined();
+
+			await DB.insertOne(database, col, newIndex);
+			// Unique index means we shouldn't be able to insert another one with the same value.
+			await expect(DB.insertOne(database, col, newIndex)).rejects.not.toBeUndefined();
+			// Unique index means we shouldn't be able to insert another one that is empty
+			await expect(DB.insertOne(database, col, { b: 1 })).rejects.not.toBeUndefined();
+		});
 	});
 };