Yes, I understand usernames are unique.
My immediate concern is usability. Login asks for Username not email address....
Ideally the actual userid would be an random unique integer. Username, phone numbers, email addresses, twitter, google, and facebook would be the unique identifiers that can be linked to the userid that the customer may never even see.
Now the user can sign in with either an email address or username with their account password.... Or via facebook, google, or twitter via Third Party Authentication.
TPA increases sales because it streamlines account creation and prevents users from having to remember yet another password. It's also inherently more secure. (no everyone agrees on that last point though)