looking for info on archaeologist/time travel novel

[ferrolad]

I'm trying to remember the title/author of a science fiction novel that I read back in the mid to late 1960s. It had an archaeologist being confronted by some folks from the future, including a woman whose was engaged to the archaeologist's identical double in the future. He accompanies her back to the future, where he wins the girl and saves the world from an alien invasion. I also remember a scene toward the end (I think it was set in Africa) where the hero and heroine discover that a professor is actually a robot; I recall something about the robot's watch not working because of the robot's magnetic field. Yes, this does sound like a silly plot, more so because of my inability to remember more than small pieces, but I do recall enjoying the book quite a bit as a teenager, and would love to find out the title/author. I don't think it was anyone well known.