Ending (spoilers)

Yes,of course it is

I watched the ending twice and I’m still a little confused. It seems like she is the double because we’re led to think we see the real Sarah being poisoned, and the fact that she was in a car accident because she seemingly was driving for the first time. But I’m wondering if the real Sarah actually killed the double and then pretended to be the double so that she could get her life back with the boyfriend and the mother? There must’ve been some significance to the fact that poisoning was mentioned earlier in the movie. Maybe she was crying at the end because she didn’t like being trapped in the role of pretending to be the double?

All through the movie I kept wondering why the actress was delivering her lines so robotically, I thought she was just a bad actress, but at the end wondered if that was to throw us off so that we wouldn’t be sure who was the real Sarah. I also kept wondering what was the point of showing the scene at the beginning with Theo James’s character and his duel, but maybe the beginning and the end mirrored each other because he was also pretending to be the double and that was why there were tears in his eyes?

I don’t know. Just my confused two cents worth.

Ha, how is there any confusion here? It couldn't be more clear, we see the entire series of events unfold with zero mystery. The double tricked Sarah into fleeing with her, whereupon she makes sure Sarah drinks heavily from her poisoned water bottle, killing her. She then takes the car and gets into an accident, because she's never driven before, and limps to the duel, poorly (and hilariously) explaining why she got into a car accident. As if that's not enough, her boyfriend and mother both talk to her about how she'll pretend to be Sarah, even going into detail about how she needs to make sure to remember to put in her brown contacts, as she has blue eyes and Sarah had brown.