Hhm, any recommendation would be helpful, a lot of the tool on this list do not actually find duplicate files. Some are plainly unusable for large file bases, such as Duplicate Files Searcher 2.2.
Usability, configurability and reliability is a key issue for such a tool.
This program, Grupa Dupa, is now available. It's faceless and appears to be an overgrown AppleScript.
I ran the demo and it took less than a minute to scan my hard drive. Although it claimed it found 3084 duplicates, in demo mode, it only lists the first two. Not a good test run of its capabilities.
After it finds the files, it leaves it to the user to find them and delete them, which seems only marginally a help.
Still, the developer wants only 99 cents for a license so it's a pittance to see how good the full product is.