Harjit Singh Ubhi

I came across a site where the goal was to block multiple login (same user id) at any given point of time for a paid website, which is very unlikely behaviour as far as the technical side and the login trend of secured login based sites goes.
we have build the site in Php/MySql and we have been successful in blocking multiple logins at the same time across the globe.
I would like to hear if anybody else has implemented this system on any of their website.