Might this be The Duplicate (1988) by William Sleator?
Apparently the boy and the first duplicate do flip a coin to decide who has to do what, and there does end up being more than one duplicate in total.
The main character, David, finds a device at the beach that can duplicate any living organism. After testing the device on his pet fish, David makes a clone of himself so that he could go on a date with his crush, Angela, while his clone attends his grandmother's birthday. His plan backfires because the Duplicate believes himself to be the original, and refuses to take orders. David ends up having to go to his grandmothers birthday after he loses a coin toss to the duplicate.
David's real problems begin when the Duplicate uses the device to create a clone of himself. The new duplicate is a less-than-perfect reproduction, being a copy of a copy, and has goals and desires that differ from the original David. Eventually, the second duplicate turns on Angela and the original David, and he has to find a way to stop him. Later, he stumbles upon something that will change his life.
According to this summary, one of the duplicates also wants the original boy dead.
When David finds a mysterious machine that can copy living things, he thinks his problems are over. Now he can be in two places at once: at his grandmother's and out on a date. While the other David is in school, the real one can spend the day at the beach. The possibilities are endless. And they turn terrifying. David's duplicate has a mind, ideas, and desires of his own – and one of them is to see the real David dead.