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.

http://www.marchanddetrucs.com/
http://www.onehorseshy.com/
http://struck.bigcartel.com/
http://www.weieast.com/
http://goincase.com/
http://www.furioustees.com/
http://www.bloomingdirect.com/
http://www.airfix.com/
http://www.abercrombie.com/anf/index.html
http://www.boredofsouthsea.co.uk/
http://www.cosmicsoda.com/
http://www.bustedtees.com/
http://etnies.com/
http://www.evelmerch.com/
http://www.imwalking.de/
http://www.letsbuyshoes.com/
http://portokal.bg/
http://threadless.com/