MongoServerError: E11000 duplicate key error collection: app.users index: email_1 dup key: { email: "ada@example.com" }

Node.js · updated 2026-11-04

Thrown by insertOne, insertMany, updateOne with upsert, and Mongoose save() when a document would repeat a value in a unique index. The message includes the collection, the index name and the duplicate key value.

MongoServerError: E11000 duplicate key error collection: app.users index: email_1 dup key: { email: "ada@example.com" }
    at InsertOneOperation.execute (/app/node_modules/mongodb/lib/operations/insert.js:51:19)
    at async createUser (file:///app/src/users.js:18:3) {
  errorResponse: { index: 0, code: 11000, keyPattern: { email: 1 }, keyValue: { email: 'ada@example.com' } },
  code: 11000
}

Quick fix

Read keyPattern and keyValue on the error to see which field collided. For signups, catch code 11000 and return a 409. For create-or-update logic, use updateOne(filter, { $set }, { upsert: true }) or findOneAndUpdate with upsert so MongoDB decides atomically.

Check err.keyPattern and err.keyValue for the field and value.
Catch err.code === 11000 and answer with a clear 409 instead of a 500.
Replace "find, then insert" with an upsert so concurrent requests cannot both insert.
Normalize values before saving (lowercase emails) so near-duplicates are caught consistently.
If the error names an index you did not expect (for example on a field that is often null), check the index definition: a unique index treats missing values as null and allows only one.

typescript

import { MongoServerError } from 'mongodb';

try {
  await users.insertOne({ email: email.trim().toLowerCase(), name });
} catch (err) {
  if (err instanceof MongoServerError && err.code === 11000) {
    throw new HttpError(409, 'An account with this email already exists');
  }
  throw err;
}

// Create-or-update without a race
await subscriptions.updateOne({ stripeId }, { $set: { status } }, { upsert: true });

Why it happens

Double submits and retries

Repeated form posts, client retries and duplicate webhooks insert the same document twice.

Check-then-insert races

findOne followed by insertOne lets two concurrent requests both see nothing and both insert.

A unique index on a sometimes-missing field

Documents without the field are indexed as null, so a second document missing it collides. A partial or sparse index fixes that.

A leftover index

An index created long ago (or by a Mongoose schema) on a field that is no longer unique keeps rejecting writes until it is dropped.

Other fixes

Use partialFilterExpression on unique indexes for optional fields.
insertMany with ordered: false keeps inserting the rest of a batch and reports duplicates at the end.

Same error, other wording

MongoServerError: E11000 duplicate key error collection: app.orders index: _id_ dup key: { _id: ObjectId(...) }
MongoBulkWriteError: E11000 duplicate key error collection
E11000 duplicate key error index: email_1 dup key: { email: null }

Let your coding agent catch the next one

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