GrupaDupa can find duplicates on any folder or hard disk very fast.

GrupaDupa creates a list of all duplicated files found. Choose an item from the list and GrupaDupa will show you each duplicate and its path. Click Skip and the list represents without that file. Select the duplicates you want to trash or to examine in the Finder. If you decide to trash the selected files GrupaDupa will move them to the trash. Warning: If you move them all, the duplicated file will also go and GrupaDupa does not remember the original location. The operation cannot be undone.

Limitations: Free demo finds and shows all duplicated items but limits the number of workable ones.

Requirements:
OS X 10.5+
Intel