I use this. It's free & very easy to use. It will detect the same image with different file size, resolution & file name or vary the level of similarity right down to at least 80%, beyond which even a blind man could tell the difference. You can tell it to look within just 1 folder or between more than 1 sub-folders within a main 1. For a massive folder just leave it running in the background. You can automate it to auto-delete or check each dup before deleting. Saves a hell of a lot of time for me.
http://www.prismaticsoftware.com/dup...pdetector.html