Idem is a very simple and crude tool which just scans the directories you give up looking for (possibly) duplicate files. This basically does so by comparing the filesize and their md5 hash.
It's not the best of methods, I know, but mostly the outcome should be pretty reliable. If you plan to kill dupes, you should check first if they are really dupes. I do not take any responsibility for files falsely marked as dupes.
It's not much, but if you have a use for it, then you may distribute it under the terms of the GPL