quicker than hashing the file, you might want to compare exif data (extract with exiftool), i have been comparing image date/time (to the second) as tagged by the camera, and when i find a duplicate, i keep the one with the largest image size. I've not worked out how to deal with those without exiftags. I understand ShotWell hashes the thumbnail to find dupes. The security camera software motion has some image comparison and to determine if the camera image has changed since the last image, i think it was visual in nature, rather than hashing, since webcams are "noisy".