The Other Me (2000)

Will Browning is a slacker student who needs a good grade in science or he's headed to a military camp for the summer. He orders a product called "Ocean Pets" for his project, but some clone formula accidentally leaks into the package. So when he stirs the mixture with a comb, he accidentally creates a clone of himself. When he discovers that the clone is smarter than him, he sends it to school--and everybody prefers it to him.