FYI, you can download your photos in bulk with Google Takeout, but you need to have enough space in Google Drive to do it. (Takeout zips up all your photos and will drop 10GB chunks in Drive.)
I was doing something similar to you recently. I downloaded all my photos and de-duped by generating MD5 hashes for all the pictures that were downloaded. (I was moving all of my photos to a local NAS, so it wasn’t quite what you are doing.)
If your dups have consistent MD5 hashes, that might work for you but it’s hard to say.