Coincidentally, I read a book yesterday which had multiple twists, most very well-executed and logical, but then couldn't resist throwing in a "Maaaaaaaybe it was the other twin all along! PSYCH!" and then never saying if it was or wasn't.

This reminds me of Isabelle Holland's Trelawny, in which twins switch places so often that by the end no one had any idea who had originally been which.