I have this repeating import problem with several hundred photos. It began on Oct 30, 2020. I see that lots of people have the problem, and there are only a few suggestions of how to fix it. Well, I don't know how to fix it yet, but I did figure out something about the problem.
The pics that do repeat certainly do keep their original long filenames, rather than something short and logical like "2050.heic, 2051.heic," etc. I think that's a red herring. What I noticed is that every repeating photo lacks gps info. I've tested by importing new photos with confirmed gps coordinates, and they are not added to the endless repeat list. I also tried to edit one older photo that is repeating, and it imported again without the added gps coordinates.
Something changed on Oct 30, 2020 and I have no idea what it could have been. I wouldn't have noticed this at all, but I just returned from a vacation and imported hundreds of photos from an iphone XR. I was grabbing my phone and quickly snapping pics so a sizable number were grabbed before the gps resolved my location. I also found a few photos with faulty locations, but those don't seem to be repeating.