Where the twist doesn't work (spoilers)
I went back and reviewed the scenes where she sees and follows herself along with the accident. I think a big problem with the twist that we're supposed to buy is she isn't acting anything like the dopplegangers in the car right before she crashes. She seems really emotional and upset by whatever just happened, like she's on the verge of tears. The she looks in the mirror, as though she's regaining her composure, and crashes.
So the problem is that she was acting human before the accident. A good twist shouldn't rely on deceptions born of inconsistency. The demeanor that she assumes once her memory returns in the bathroom is the one she should have been wearing in the car.
I think there was a really cool idea behind this movie, but it wasn't executed in a particularly compelling fashion. The mirror dopplegangers and the meditations on identity were ultimately only there as props while the movie pretty much existed solely to have a plot twist. Basically it was just: Exposition, Weird Crap, Plot Twist, Fin.