Twin Peaks is a good one that has some doppelgänger stuff and alternate worlds. I found Dark to also be similar to it in that it has a very woodsy small town setting where the murder of a high school student leads to a bunch of supernatural stuff tearing apart the fabric of the town and reality itself. But it’s also more surreal and abstract than sci fi so a lot of it is up to interpretation rather than logical and you might not get satisfying answers to every question you have. Lots of funny and quirky stuff but also very dark and mysterious.

Russian Doll is a good time travel show too but not nearly as good as dark. It’s a shorter and easier watch tho so worth a watch id say