Eventually found all the duplicates in my migrated iPhoto library...

First required going into preferences and updating the path to the iPhoto library (because file name had changed from "iPhoto Library.photolibrary” to “iPhoto Library.migratedphotolibrary”. This was done by just clicking on the grey iPhoto icon found in preferences and then navigating to the migrated photo library. So far, it has been unable to find any *EXACT* duplicates in the Photos Library.photoslibrary, but I suspect it is because some went through file size optimization using Google Photos, and of course SHA-1 Hash function will not “see” these two (nearly) visually identical files as being identical. So, that means that I will be going through the library and finding duplicates manually. Ugh.

davidgrantaustin1 about Duplicate Photos Cleaner