E11000 duplicate key 🟩 Node.js

MongoServerError: E11000 duplicate key error collection: db.users index: email_1 dup key

An insert/update violated a unique index in MongoDB.

Seen on: Node.js

Meaning

The message names the index and the duplicated value. A dup key of null means several documents lack the indexed field (unique indexes treat missing as null).

Common causes

  • Duplicate value for a unique field
  • Missing field on several documents (dup key: null)
  • Leftover unique index from an old schema

⚡ Quick fix

  1. Handle code 11000 as "already exists" (409)
  2. Use partial or sparse indexes for optional unique fields
  3. Drop obsolete indexes

Detailed fix by platform

Node.js

  1. catch (e) { if (e.code === 11000) return res.status(409).json({ error: 'Email already registered' }); }

How to diagnose

  1. Index — Which index?
  2. Value — Which value (null?)

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