I apologize for not being clear. I am looking at a particular contact, and wanting to add or change a photo. I want to select a photo from my Mac's photos library. I do so, selecting "people". Going down on the list of people, I find two duplicate names (my son and his youngest son). Looking at the photos of those names, I see that they *are* photos of the people, but not the same photos. So I opened the Photos app. I opened "people", got rid of hearts, and sorted all of the faces in alphabetic order to verify that they weren't duplicates. There aren't.
Here's a separate problem which may be related:
Just now, I went to Finder on Catalina with my iPad plugged in. My iPad synced with "Sync Photos to your device from 'Photos'", which is what I want. After the sync finished, I opened Photos on my iPad. The iPad asked me if I wanted to use the iCloud, no I tried that in the past, I want to sync with my Mac. This has 65 people, including 3 people with hearts and some that were deleted from my Mac. My Mac has 97 people. I started looking at my "people" photos, then backed out for a moment. When I went back in, I saw a message saying "29,715 Photos Scanned To finish scanning your remaining 1,601 photos, lock your iPad and connect it to power." Hmm, it was locked and connected to power all night! My son's face is missing. I did combine 3 "faces" of my grandson. I will lock my iPad shortly.