Check for user.isonline before automatic login?

Question

Hi All,
I'm trying to stop multiple users logging on  to my site using the same  user details. I have so far managed to run a test in the LoggingIn event on the login panel against the membership database to see if the users "inputed" username is online using user.isonline(username).
This works great and set userIsOnlineTimeWindow in the config to 10. I then stop them logging in, display message and log the Ip and computer name to see if anyone is persistently attempting access the site.
So far so good apart from the users who have saved their login details as a cookie through the remember me.
I basically want to read the username stored in the cookie and again test to see if online before the application automatically logs on the user.
Is their a membership stored procedure I can call to sent the encrypted cookie details to? or way I can achieve this in a fairly simple way?
Any help with this would be much appreciated.
Jeremy