After deleting a user (send to trash and empty trash), it's impossible to create another user with the same username.
- Create a user (username: john, password: publish, email: email@example.com)
- Delete this user (Content object is sent to the Trash)
- Go to the trash
- Empty the trash
- Go back to Users in Admin Panel
- Create a new user using the same values as step 1
It seems that the ezuser database table still has the entry
Website administrators should be able to create a new user account using the same username if the previous account is not in the trash.