Google Photos Takeout and Android Photo Overlap Issues

I downloaded all my Google Photos, and they were organized into folders by year. However, on my Android device, I still have photos from my camera, screenshots, WhatsApp videos, and more. I'm planning to sync them using Syncthing, but I'm concerned I'll end up with hundreds of duplicate photos on my phone.

Has anyone else had this problem? How can I keep my Android photos separate from the Syncthing ones? My plan is to keep all the photos already on my phone and only sync the new photos from Syncthing that I don’t already have on my device."