howto get user window status question

[eluser]Suanko[/eluser]
I have about the same question about status/session :

I made a database both for my app user list (users table) and for the sessions (sessions table).

I don't want 2 people to connect both with the same login/password.

What happen if a user close the window without logout first? Next time he will connect, how can i avoid the system to reply "you're already connected"? Shall i check the session table for user_data->status in the login script?