What the best way to.....

[eluser]limit[/eluser]

If your already using sessions: When a user logs in, you can just make a function to pull the username from the user table and then store it in the session.

(eg: select username from users where user_id = $user_id

Why your able to login to both admin and the user side depends on how you setup your database and code. There is not enough info to help you with that.