I have used Duplicate File Finder to find duplicate photos on my hard drive. It’s pretty slow but it does work. If you have a lot of duplicate files, it’s output isn’t perhaps all that helpful. It’s biggest help is perhaps showing you which folders contain some of the same files and then you can go from there.
What I find is better and much faster is just to try and combine everything where it belongs. What I do is use SuperCopier. SuperCopier will ask you if you want to overwrite a file (and if you should do it once or for all). In this way, you can merge non-destructively two or more folders. Then when I know all the contents of the other folder has been merged into the other, I delete it.
This doesn’t help you much if lots of names have been changed and that’s where Duplicate File Finder comes in handy. But I have to say in most cases, I’ve found the copy method to be far more efficient than waiting hours or days for Duplicate File Finder to run only to find out that yes, a zillion files here are also there.