Find duplicate images with SimilarImages
SimilarImages is a free program that will let you discover duplicate or similar images on your PC. It will scan folders you will define.
You can set a threshold value that determines how much alike images need to be in order to be considered similar.
Once the program has finished scanning your image folders, it presents you with a side-by-side view of all similar images, to let you choose what to keep and what to delete.
The program stores image information in a database along with your own decisions, making subsequent searches faster.