The devs can see orphaned photos e.g.
https://forum.inaturalist.org/t/not-all-observations-uploading/4946/37?u=karoopixie
and they fixed this problem (see the thread referenced above), which means that orphaned photos that didn’t load an ob will now be loaded, and one will therefore have duplicates.
The best way to search for duplicates (that I have found) is to go to Your Observations and arrange them by Date Observed. You then just have to scroll through the whole lot to find duplicates which will now be next to each other.
But perhaps this doesn’t answer your question at all and I have pootled off down the wrong garden path altogether.
Off-topic: I tried to quote @kueda 's comment above, but it seems that one can’t quote closed topics. Or maybe I need to eat lunch first ;-)