So I remember reading this book in the early 2000's, and I'm pretty sure it was placed in my classroom, so it was obviously geared toward young adults. It was about a girl who wakes up one day with a birthmark (I think it might have been crescent-shaped). At some point, it is revealed that she was either cloned or genetically engineered and that there are several duplicates of her, all with the same birthmark. Its possible that the bad guys (who cloned/bred/grew her) want her back and try to take her from her parents, but I'm not sure. One thing I do remember is that the girl finds one of her twins or duplicates or whatever, and she's a Russian ballet dancer. I think it was a pretty short book, part of a much longer series that I couldn't find at the time, and I'd love help digging it up again.
Replica by Marilyn Kaye.
From the wikipedia page (the title is the link):
"Replica" is a children's science fiction series about Amy Candler, a young teenager who discovers she is one of thirteen cloned girls who have been genetically modified to have superhuman abilities. She was created in the government-funded Project Crescent, and was kidnapped and adopted by one of the scientists after a change of heart. Throughout the series Amy struggles to live a normal life as the mysterious Organisation tries to use her and the other Project Crescent clones to create a master race and take over the world. [...]