No two users can have the same username.
Make sure you pick a unique username for each user you make. If you try to give a user a username that already belongs to another user, the management interface will refuse to create another user.
Once a username has been used, it can never be re-used.
Even if you deactivate a user, you cannot re-assign its username to any other user, ever. Choose wisely.
You cannot change a user's username once you have set it.
Again, choose wisely.