The movie begins with Gina (Lena Headey of TERMINATOR: THE SARAH CONNOR CHRONICLES) and her boyfriend attending the birthday of her father, who is an American who works at the U.S. Embassy there in London. Also there is her brother and his girlfriend. During their dinner a huge mirror falls off the wall and breaks, for no apparent reason.
Things start to get weird when she's making a phone call and sees herself drive by, in her own car. She follows the car to a parking lot and follows the woman up to her apartment. Then, the next thing we see is that she's upset and driving back in her car--and gets in a bad car accident. She recovers but has only fragments of memories. And she begins to think that her boyfriend really isn't her boyfriend anymore, which may indeed to the case. Or is it because she has head trauma from the accident.
This is one creepy, suspenseful movie and one of the best movies on doppelgangers I have scene, though a very similar idea was explored a decade previously with J.D. Feigelson's television movie, THE LAKE. Highly recommended.