Hi @noelt.dolan,
Welcome back to the Auth0 Community!
What I would recommend in this case is to firstly check if the user exists using the GET /api/v2/users and GET /api/v2/users-by-email from the Management API. If you are not able to locate any instance of the particular user in any of the API calls, you can try to delete the user by email using the DELETE /api/v2/connections/{id}/users endpoint which should remove any reference of the user.
Additionally you can also check this article on the matter - “User Already Exists” Error When Creating New User, which presents a few troubleshooting steps.
Please let me know how this goes and if the issue resides afterwards as well!
Thanks,
Remus