How to restrict single login with in portal

You need to implement Single SignOn. You can manage current logged in sessions in database and on login, see if that login is active. On logout, remove the active session (user id) from the database. If a user who is already has an active session tries to login, show a message and ask to kill the previous session before user can login with the new session.

Dear

        You cant find who is current login  so use data base in add flage in login table and set at login time true and false at logout time so you can stop to login same user at time one.