Firebase Auth errorFirebase Auth: auth/credential-already-in-use

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Why the link fails, and the fix

1. 01

   An anonymous guest signs in with an existing account

   “Account linking will fail if the credentials are already linked to another user account.” Firebase, Link multiple auth providers

   The guest is one Firebase user, and the Google account already belongs to another. Firebase won’t join two users by linking, so the app has to switch to the existing one and move the guest’s data.

   import { getAuth, linkWithPopup, signInWithCredential, GoogleAuthProvider } from 'firebase/auth';
   
   const auth = getAuth();
   const guest = auth.currentUser;
   try {
     await linkWithPopup(guest, new GoogleAuthProvider());
   } catch (error) {
     if (error.code === 'auth/credential-already-in-use') {
       const cred = GoogleAuthProvider.credentialFromError(error);
       const guestData = await loadData(guest.uid); // your own data layer
       const { user } = await signInWithCredential(auth, cred);
       await mergeData(user.uid, guestData);
     }
   }

2. 02

   The guest data is read too late

   After signInWithCredential the app is signed in as the other user, and rules may stop it reading the guest’s documents. Read and keep the guest data before you switch.

3. 03

   The old guest user is left behind

   Each failed upgrade can leave an empty anonymous user. After the merge the app is signed in as the other user, so delete the guest from your server with the Admin SDK, or let a scheduled cleanup remove old anonymous accounts.

4. 04

   A signed-in user links someone else’s account

   Also check: on a shared device, a user can pick another person’s Google account in the chooser. Show which account is in use and let the user choose to switch rather than merging silently.

Checked against the vendor’s docs on November 9, 2026: Firebase docs: Link multiple auth providers.

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Related: Firebase auth/provider-already-linked · Firebase account-exists-with-different-credential · Firebase auth/email-already-in-use · Firebase phone auth invalid-app-credential