If I create 2 'Brands' like this:
Brand 1: ABC with username: hello@abc.com
Brand 2: XYZ with username hello@xyz.com
The client (owner of brand 1) can then click 'Settings' and change his email address to hello@xyz.com. Both usernames are now in the 'login' table with no disambiguation. Perhaps brand-owners should not be allowed to change login name?