Firestore errorFirestore: “6 ALREADY_EXISTS: Document already exists”

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Why the document exists, and the fix

1. 01

   create() only writes new documents

   “Create a document with the provided object values. This will fail the write if a document exists at its location.” Google Cloud Node.js client, DocumentReference.create()

   In the Admin SDK, create() fails when the ID is taken, on purpose. batch.create() and transaction.create() work the same way.

2. 02

   You meant to overwrite or merge

   “If the document does exist, its contents will be overwritten with the newly provided data” Firebase, Add data

   set() creates the document if it is missing and replaces it if it exists. Add merge: true to keep fields you don’t send.

   await db.doc(`users/${uid}`).set({ plan: 'pro' }, { merge: true });

3. 03

   A retry ran after the first write worked

   A function that retries after a timeout can run create() twice. The first one wrote the document, so the second fails. Treat code 6 as success when the document is the one you meant to write.

   try {
     await ref.create(data);
   } catch (err) {
     if ((err as { code?: number }).code !== 6) throw err; // 6 = ALREADY_EXISTS
   }

4. 04

   The ID isn’t as unique as it looks

   IDs built from an email, a date or a name can repeat. Use add() or doc() with no ID for a random one, or keep the natural ID and expect the error.

Checked against the vendor’s docs on November 9, 2026: Google Cloud: Firestore Node.js DocumentReference; Firebase docs: Add data to Cloud Firestore.

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Related: Firestore transaction too big · Firestore unsupported field value: undefined · Firestore query requires an index · Firebase “Missing or insufficient permissions”