The book is about a boy who I believe goes to a beach and finds a mysterious machine. He brings it back to his room and puts his fish in it, and the fish suddenly duplicates. He eventually duplicates himself, and he and the duplicate alternate going to school. Eventually the duplicate duplicates himself. The original has a girlfriend but the duplicates also spend time with the girl, the original becomes jealous and tries to control the situation. He looks at his goldfish and sees that one of them is dead. He realizes that each duplicate is more evil, or 50% less moral, or something, instead of perfect duplicates. The first duplicate is friendly and meek, but the second duplicate is evil. The original pairs up with his girlfriend and by accident the two duplicates fall to their death. It's a young adult novel and for the life of me, I can't remember the title.

Thanks for your help!

The Duplicate by William Sleator: Things go entirely awry when David discovers a machine capable of duplicating organic matter and uses it to help him appear in two places at once.

Also --

When David finds a mysterious machine that can copy living things, he thinks his problems are over. By duplicating himself, he can visit his grandmother and keep his date with Angela. While the other David is in school, the real one can spend the day at the beach. The possibilities are endless... and terrifying. What begins as the solution to David's problems soon turns into a nightmarish struggle for identity. Then events take a horrifying turn that neither of the two Davids can control -- and that may have deadly consequences.

That's exactly it! Thanks!