Cool Sci-Fi flick from the 60s and my childhood

I remember watching this now-forgotten futuristic science fiction movie on television back in the early seventies. By then this movie was only some ten years old. I remember the cool-looking androids, basically stuntmen wearing rubber masks that by today's Star Wars standards would be goofy-looking. The mutant men were pretty fierce and scary-looking, given their skinhead looks, scarred make-up and the fact that it looks like only six-footer stuntmen were selected, making the mutants look bigger and meaner.
This sci-fi flick ends on a disturbing note. At first it looks like our intrepid team of scientists escape with survivors of the future into the future. But some kind of time loop is inadvertently created, causing infinite copies of the scientists to go through the same set of events, ad infinitum. Spooky.