On Xbox Series X you see twins on the streets from time to time, but that's it. Doesn't bother me too much. But THAT is a little extreme.
I found that the game spawns duplicated npcs a lot when loading a save file in the middle of a densely populated area, or when you purposely turn your V repeatedly around to force npcs to appear. That's because the game is forced to load on the spot a lot of characters, and some of them end up being clones. If you just roam around it happens much less, because the game has the "time" to load npcs in a more gradual and natural way. But when it's forced to load a lot of things on screen at the same time, then things can go funky.