Three friends, Tracy, Craig and Derek, go camping to isolated Brewster Island. As soon as they arrive Tracy has to go to the bathroom. Instead of going out in the woods, she breaks into the only house. No one is home. That night, there's a bad lightning storm so they flee their tents and take refuge in this same house. They take showers and make themselves at home. Then, the owners show up in the morning and they flee into the basement where they discover two dead bodies.
They are discovered by the owners, an older married couple (the guy is John De Lancie from ST:TNG), and are forced to bury the bodies. But before they do so they see that the corpses are duplicates of the killers. But before they are slain, their own doppelgangers show up and save them.
This is all because of a secret lab from the 1950's. A scientist was working on cloning and was killed by his own creation. The house's plumbing leaked into the lab, so their DNA--from the showers and toilet--prompted the cloning process to begin. These clones are stronger and more aggressive--and seem to possess no conscience. It's just a matter of time before the clones of Tracy, Craig and Derek kill the originals. This is complicated by the fact that clone Craig is attracted to original Tracy and original Craig is attracted to the Tracy clone.
This is an excellent sci-fi/horror movie with an extremely satisfying ending. Highly recommended.