+1 for Duplicate Annihilator for Photos that Mike mentioned. I used it on a neighbors Photos library and it worked quite well. Don't get it from MacUpdate though since they have been bundling their downloads with adware lately. Get it directly from the developer.
Gemini will find all duplicate files on your system. I do not keep image files in the Photos library, all mine are referenced images, so I have not tested that but I do know it finds other duplicate files.