After something strange happens in the secret government facility where he works, Dr. Gerald Appleby is surprised to find that someone has stolen his car. The guard looks confused when he asks who was driving his car, since it was Appleby. Back at the office, his secretary asks him if he forgot something since he was just there. And when he shows up at his girlfriends house, she calls the police to have him arrested since his clone is already at the house.
Soon Appleby figures out what is going on. The facility he works at is cloning frogs, and they must have somehow cloned him. But everyone he encounters believes he is the impostor. The police are after him. The government is searching for him and want him dead. And his girlfriend, still clinging to his clone, thinks he's a weirdo.
There's never any explanation for why people aren't more confused by seeing two men who look exactly the same and who are both claiming to be Appleby. If your boyfriend showed up at your door when you are already with someone you believe to be him, would you assume the new arrival was an impostor? Or would you question why there are two people who look exactly like your boyfriend in your living room? It seems more likely that you'd think you were dreaming or hallucinating, than to automatically assume the second guy must be an impostor.
There's also no real explanation for why they've chosen to clone Appleby, or why they'd want to kill him and replace him with his clone. The clone has the same memories and thoughts as Appleby, so it's not like they are replacing him with someone who will do their bidding. Even with those plot holes, I liked the film. While the cloning is the center of the storyline, it's more about Appleby and how he responds to what is happening. There are multiple scenes with the government goons chasing Appleby and trying to kill him, but he turns out to be pretty wily.