If you sign people up more than once, with multiple authorities, they will have multiple local accounts, but the autority data model and authentication code have no knowledge and could have no knowledge of how these users are mapped except by the username,authority_id connection.
I think what you are proposed is seperate from what the current authentication package was designed to do. It is not a flaw or a bug, it just was never designed to do that. Anything that changes that needs to make sure it doesn't change any other behavior or break any existing use cases.
