I've been doing some cleanup on my ReadyNAS due to it running low on space. And for now, I have it back to a manageable size, but I know more can be done. Over the course of the years with various backup schemes, I know I've probably duplicated some files. So now I need an easy way of finding them. Anyone know of a good way to do this on an OS X box?

Ideally, I'd want to tell whatever program/script what paths to search, and I could have it look locally as well as on the NAS and inside disk images by having them all mounted prior to the check. It would be nice if the program just initially relied on filesize, then CRCs on files of the same size to sort out false dupes.

This is one of those programs that is hard to find, because Google is full of hundreds of unknown Windows utilities, even when you try to specify Unix or OS X only.