RDC connection blocking

HI

I have an app that runs on an XP box that is accessed over the network via RDC, it al works fine enough in this scenario. However I need to block a 2nd user logging in if it is in use. Currently if someone else tries to login then the current user loses their connection and we have RDC tennis on our hands. I need to have a message pop up to the 2nd user if it already in use, I know it can be done as I used to have it in place but after a server rebuild it somehow seems to have lost this facility and I can’t remember how i did it before(I set it up about 2 years ago).

Thanks