Is there a reliable way to determine if two or more photos are the same, regardless of relative image size differences?
I am going to steal @osullic comment.
If so, I have used VisiPics (on Windows) to do what you want.
I have tested it a bit and it is doing a decent job. It is a really small application for windows. It can be improved a lot, but it works.
It marks as green if it feels it is an exact duplicate, and some other colors like orange if it is just similar.
Another option: duplicate-finder.com It assumes the bigger file is the original, but it makes a good job.
This two, unfortunately, seems to be discontinued.
Another that does the job is pixiple.
There are some other alternatives, which I still need to test. https://alternativeto.net/software/duplicate-photo-finder/