Currently, if you open a torrent with LimeWire, e.g. from a browser, a second instance of LimeWire pops up. Clearly LimeWire is the kind of application that should be single-instance under all circumstances, since running two instances of LimeWire causes odd effects in your dowload list and other places.
So in short:
a) Open the torrent in the current instance of LimeWire if running, rather than start a new instance.
b) If LimeWire accidentally gets started twice, it should inform the previously started instance of its command line arguments and terminate. This will add robustness.