K. Dawn Byrd's "Double Identity"
Bree soon discovers that even though they look alike, they're totally different. Cassie is wild and impulsive. She hates Bree's little town and everything in it, except Bree's boyfriend, Luke. When Cassie becomes obsessed with Luke, she'll go to any length to have him for herself.
Luke has a secret, which Cassie learns and uses against him. She's off her medication and will stop at nothing. She says he's in love with her. He says he loves Bree. Will their secrets destroy them and their relationships?