Through the decades there have been wonderful (not classic, but fascinating) movies about twins. Bette Davis starred in two. Both great hits at their release.
And she managed to get double billing!
One of the best of the genre is The Dark Mirror, which helped catapult Olivia deHavilland back into stardom after a few years off the screen because of contract disputes with Warners.
And two “small” films are worth noting. Monogram made a very “B” Dark Mirror rip off, with Bonita Granville.
And, while technically not about twins, Barry Nelson’s performance as a man and his doppleganger, is riveting in The Man With My Face