Counting users logins on separate devices

In your user login table store the session id of the user logged In. When user logs out destroy the session Id and remove session Id form login table. If some one else wants to logIn at the same time when some one is already logged in with the same username or password check for the session Id in the table if there is any session Id already in table block from loggin other wise allow

Thank you for your input.
I may be wrong but this solution has a loop hole because users may not manually log out. They can just close the browser and the sessions may time out. If that happens there is no trigger to update the database of that user logging out causing an in accurate count.