Session persistence problems

[eluser]WanWizard[/eluser]
Very difficult to archieve, since you're working in a stateless environment.

Some have modified the session library to include a userid field in the session table. Others use a regex to check a session with specific information in the session's userdata.
But that still doesn't give you any clue to what the users actual state is. If you're session expiry is set to 2 hours, and I close the browser on my PC, walk to another PC, and try to logon, my previous session is still valid (and has me as logged in), but for me I'm not logged in anywhere. It's very confusing for users.

Unless you have a very specific reason to want this, why bother how many times someone is logged in?
You have to deal with concurrency anyway. I can also open a second window in my current browser, so I have access via two windows. Why should that be any different from me accessing the site via both FF and IE from the same PC at the same time?