It seems to me now that the folks who thought up “multiple parallel universes with the same people in them” were playing with the old Stoic idea of a Grand Cycle of universes being created and destroyed, except that each one would pop up again exactly the same, with the same people. (Or with people who looked and acted and were named exactly the same, even if they weren’t the same souls.) Parallel universes are the same exact thing, except without all the creating and destroying being necessary, and with a little more artistic variation.
Stanley Jaki’s book on science and civilization goes into this Stoic idea of the Grand Cycle stuff in great depth, and it really does become immediately obvious! Explains a lot about alt-history memes, too.
(Darn all those classically trained early sf authors, who thought we would recognize this stuff without being told!)
There’s a lot of fun in ficition about parallel universes, but as a scientific idea it’s pretty well crap. It is being used to spackle over problems with people’s favorite theories, basically. Even as fiction, it has a lot of limitations, as Larry Niven has copiously explained. (People like their actions to matter, not to have every anguished decision be reversed in another leg of the trousers of time.)
There are ways that one could make it work, but the “alternate you” would have to be a totally separate soul, with totally separate free will. An other-universe twin, as it were. And so would all the other alternate-people have to be separate. So you really couldn’t have any close parallel, because everybody’s life is full of decisions, great and small. The only thing you could guess is that if you don’t like getting invaded, you don’t want to live in Belgium or Ukraine or the other natural invasion crossroads,
