As requested by a commenter to this hint, I've converted the bash script there into a droplet. Drop a file on it, and it will use your Spotlight index to see if you have a potential duplicate somewhere on your system. Download the free DupeCheck droplet, unzip it, and find a good spot to put it. Drag and drop any file onto it, and it will use Spotlight to identify possible duplicates.
Thanks again to the bash gurus on macosxhints.com who vastly improved my original script (full history and credits available in the original hint and its associated comments, linked above).
[robg adds: This works quite well, and makes the Unix script solution from the original hint usable by everyone -- no Terminal knowledge is required. I have also mirrored the droplet on macosxhints.com, but you should grab the original if possible, as it likely will be newer than the mirrored copy.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061101221400774