Many of my photos are duplicated. I think this happened because before there was the option to sync Drive photos, I uploaded my photos using Backup and Sync (on Windows), choosing the option to simultaneously upload my photos to Photos, using compressed quality. Then a year later when the 'sync Drive photos' option was enabled, I synced my Drive photos, but ended up with loads of duplicates. Photos obviously should not have presented this problem, as it should have the same hash for the duplicated photos. (I'm uncertain whether this problem with uploading using Backup and Sync as well as syncing Drive with Photos has been resolved.)
It is difficult to determine why there are two copies of each photo, but one reason could be that I initially uploaded in the compressed quality, but the photo in Drive is in original quality.
I have consulted https://support.google.com/photos/thread/196108?msgid=196782 , which basically recommends to look at the upload date, but I don't want to delete whole batches of my photos by date, as I may end up deleting photos that aren't duplicated.
In the absence of a tool to resolve duplicates, the safest option is for me to compare each pair individually, but I can't see any difference between two photos when I compare their info in Photos. Ideally, I want to see where a photo is stored when I click to see the info about the photo (but I can't). I thought that if I add the Photos folder to My Drive, I would be able to see this information, but very few of my older photos appear there (only about 5% of my old photos), so I couldn't see duplicates that way. The photos in this folder do not take up any storage as far as I can tell, and only display the folder location of the Photos folder they are in. (It would be better if all my Google Photos were there, even if it meant some photos simultaneously have two different folder locations.)
When I search a duplicated photo by name in Drive, it only returns one photo! The storage used on this photo is zero. If the problem was that a compressed photo in Photos duplicates an original photo in Drive, I would not have expected to not be able to find the original photo in a Drive search.
I do not know how to decide which duplicate photo to delete from each pair. I don't want to make mistakes, as these are the only copies of my photos that I have, so I want to be informed about which photo is poorer quality before I delete one. As I said before, the information presented to me in Photos about each pair is identical.
Note that I have cross-posted this from https://support.google.com/photos/thread/471190 because I received no answers there since I posted it two weeks ago.