Most likely the facehuggers can sense which victims have already been parasitized. I can't find anything via Google to support it, but in several books I've read on parasitoid wasps, they seem capable of sensing which caterpillars have already been attacked and prefer those who haven't (ensures a larger meal for their own offspring).
Facehuggers aren't trying to come up with a clever scenario for a movie watcher's entertainment, after all. They're (pseudo)organisms attempting to reproduce. If they impregnate an already impregnated victim, there's no way they will develop to maturity before the first one does, which probably means they just die.
Incidentally, the gigers themselves will most likely take care not to injure such a victim (indicating that they can sense that, at least). Though, they may drag them away to be cocooned in the resins. I would hazard the guess that it cuts down on heroes surgically rescuing them that way.