Single Instance of Login at a Time

Hi,

 

I have a web application, i want that one login id can login only once at a time. Let me make it clear let a user has login id called 'sujit' and it has logged in at 10:00 am in a machine, and the same user is trying to login with same id at another machine at time 10:10 am while first login is not logged out till now, so at 2nd time login should not be success. it should show a message. the 2nd login should only be success when the first one is logged out or its session is expired..

 

Thanks...