For Windows people, I recommend PzDB from Rocketship software. While it doesn’t specifically have a Find Duplicates function, displaying files in alphabetical order will certainly find a lot of duplicates.
It’s a database that scans the runtimes you inform it about and automatically adds files and categorizes them. As you can set up your own groups and add your own keywords to search on, you can set up any organization you like without physically moving a thing.
It uses the Microsoft Access engine, though, so no Mac version. :-(
I’m beta-testing version 1.3 at the moment.
Oh and Paul? Thanks for adding a Tellarite to your entourage, even if she is wearing too many clothes.