[Nadiar]

You can put a 'session' into the login cookie. Then anytime someone logs in, it generates a different session. If the Session, Password, and Username are not matching, it logs you out and deletes your cookie automatically.

You probably don't want to get into banning people for having 2 accounts logged in at the same time if they're paying for access, but its certainly possible to ban them if they login with a different IP within a certain amount of time from the last account use.

If you want to be more friendly, I would allow multiple logins as long as the IP's are in the same scope.