Even if the users are in different authorities, the username, along with several other fields (including authority_id) are in the users table.
If you make the field unique, then you will almost certainly break more things than you fix. If the username was forced to be unique, then we couldn't store usernames in that table for external authorities. If we didn't have that, where would we get the username for a user_id if we needed it? Not to mention all the views that have it as well, such as cc_users.
