The Duplicate

A Young Adult novel written by William Sleator in 1988.

A teenager named David finds a strange object at the beach that has the ability to make copies of living things. In order to go to his grandmother's birthday and attend a date at the same time, he clones himself. The clone, who is a bit misguided, ends up cloning himself as well, and the third clone... well he's of a different sort altogether.

Provides examples of the following tropes:

 * Clone Degeneration: The duplicates are less sane the farther they are from the original. They also get black markings on their hands shortly before they die, but they tend to go crazy and get themselves killed before that step occurs.
 * Cloning Blues: The clones have it rough. First off they get less and less sane the farther from the original they are, and even the sanest ones develop black marks on their hands and die abruptly. And since they're not convinced that they are copies (they're physically and mentally identical to the original until the marks appear), this all feels monstrously unfair.
 * Evil Clone: The clone isn't really evil, just resentful of being treated as a clone (he has all the memories of the original, so he believes he is the original). However, this leads him to.
 * Literal Split Personality: The clones believe this, and even convince the original for a little while, but they're ultimately wrong.
 * Mes a Crowd
 * Two Timer Date: David accidentally schedules a date as the same time as his grandmother's birthday. In order to avoid the Hilarity that would ensue if he tried to run back in forth between the two, he uses a mysterious device to duplicate himself, so that he can attend both. Then his duplicate wins the coin toss and the original has to go to the grandma party, and it all goes downhill from there.