I just saw Shutter Island with Decaprio. “Enemy”is very similar and therefore I figured it out almost at the beginning. There were hints too throughout the entire time (Mother, blueberry, mirror, dark colors, behavior of “both” men..). I liked the movie very much even I knew what was happening. The spider was the answer for those viewers who could not figure it until the very end.
In Shutter Island it took a couple of doctors explaining to the audience at the end what went “wrong”.