Apple Photos creating random duplicates

Hi

I was adding new photos to my apple photos library (quite a few thousand old photos) and then I found that there were a few thousand that said 'unable to upload'. So I did a repair of photo library and now in the middle of syncing all photos to cloud again.

However on the apple photos app on computer during the period of 2015 to 2017 ALL the photos/videos are duplicated and its only for this period. But when I go on my phone I don't see any of these duplicates.

Can anyone explain why this is? Surely I can't have done something wrong when repairing otherwise every photo will have duplicated....