Eh I just posted about this in Coding Snacks as somebody brought it up on another form this week. Bit much for Snacks I think but don't ask don't get etc...
I agree anyway, I'm surprised if there isn't already something out there that does this. Tineye etc for local hard drives, would be great.
Duplicate finders ... yes but the trouble is that they will throw up a list of all duplicates, not just the ones that relate to the pic you're working with. Unless, I thought, the finder has an option to ONLY show cross-directory duplicates in which case you just copy your pic on its own to a New Folder and then compare that with your pictures directory.
Visipics I like a lot but it doesn't do cross-directory comparison full stop. Or not for me anyway. Also, try grayscaling a picture and see if Visi flags it as a duplicate (it doesn't). Anti Twin
on the other hand does.
So for a thorough duplicate search I would use two or three different searchers as they all have strengths and weaknesses.
There's also a tempting looking prog called Photology which claims to be able to search by "simple and intuitive filters like faces, sky, color, location, and time of day". Photology
I downloaded it ages ago but still haven't got around to trying it out yet. It apparently works by indexing all your pics by some criteria or other and then ... don't know :-)