mixture has the elegant solution, using hashes.
There's a quick and dirty method (which I have used more than once):
If you are using Windows, and
IF (big IF) your files have unique names (eg IMG00000001.JPG), then tipping the whole lot into a single folder will prompt Windows to ask what it should do with duplicates. Tell it to keep one only, and it will dump the rest.
The catch is that if you have several different pix with the same filename, you'll lose all but one of them.