[Shub-Nigurrath]

everything is possible, and there are of course no limits to ways you can do to check the presence of another istance of the same program, the concept is that the two istances must share an information, even a single bit, but among the simplest method there's also the check of the main window's class name. try to have a look at it.
The best way is to check where the program decides to go on or to terminate, so try tracing exitprocess (or any exiting way) and then go backward to where it was called from..