Let's be clear: I don't actually think "The One I Love" hinges on a twist.
Instead, I think it's a very clever, very clearly laid-out fantastic premise on which to hang questions of character and behavior. There is a reveal early in the film, and another late in the film, and that's it. And honestly, telling you that doesn't ruin anything. It can't ruin anything from this film. This is a movie about behavior. It's about the delicate, fragile fabric of marriage and how easy it is to unravel once you start picking at it.