Context: #1781

The problem occurs when an already existing user that was registered locally is being authenticated by OIDC. Docspell looks up the user with account_source = openid, doesn't find anything and tries to create the user. But there is such a user already with account_source = local. It should return an error that an account like this has already been added from a different source or automatically treat it as the same account.

The other way around is handled. When trying to register an account that has been created by OIDC it sends an error message "A collective with this name already exists.".