User already exists on signup: duplicate identities and account linking

TL;DR: Symptom: signup says the user exists, but the user cannot log in. Cause: an identity already exists on another connection. Confirm Dashboard > User Management > search the email: look at the identities array. Check email_verified on each.

The fix

1. The email exists on a different connection: signed up with Google, now trying database signup. Same email, separate identities.
2. Case or whitespace variance created a near-duplicate.
3. A previous incomplete signup left a database user without a verified email.
4. Link the accounts: Management API link endpoint with the primary user_id and the secondary identity. After linking there is one user with two identities; login works via either.
5. Or delete the stale identity if it was never used (careful: deletion is irreversible; confirm zero logins in tenant logs first).
6. Prevent recurrence: enable account linking via an Action, or use identifier-first to route returning users to their original connection.

When to use this

• This covers exactly what the title says: User already exists on signup.
• You are setting this up for the first time, or auditing an existing setup.
• You want the key gotchas in one place before you start.

When not to use this

• You are doing a different workflow with User; these steps are specific to the title above.
• You need the full reference docs; this is the short path, not the manual.

Compatibility

• Not pinned to a specific version; follows current User behavior.

Auth0's user account linking is documented in the user management docs, including the Management API link endpoint and users-by-email lookup. Verified against live docs 2026-09-26.