This is because the Name must be unique. If it exists in the system, it will not allow a duplicate. Most likely, the other instance of the user is deleted and has the same name. If you change your View's filter to show Deleted users as well, you will see the deleted users (they are never completely deleted). If you go to the deleted user and change the Name, you will then be able to go to the Active user and remove the number from the name.