Windows: SimilarImages is a duplicate file finder tweaked to find image files. Rather than simply compare raw stats it also scans the photo itself. As a result you’ll find more duplicates and be able to select which file you wish to keep.
Point SimilarImages at a directory or drive and it will scan all the image files. You can tweak how thorough it is and whether not it simply compares the standard factors (file name, size, etc.) or if it actually scans the photos themselves and compares the results. On first run the app is a bit slow but it maintains a database so subsequent searches are faster–the database also includes instances where you’ve flagged a match as a false positive so you won’t have to flag it again in the future.
SimilarImages is freeware, Windows only.