For music files (mp3 etc) I find iTunes excellent.
http://www.apple.com/itunes/download/ free download. Others will have their preferences, and for other types of files the Mac which I use can do the job fairly well on it's own. The Mac will in fact search for all mp3s and list them in various orders ...
... Can windows not do this?
iTunes will give you a separate list of duplicates among many other things, but the tunes need to be loaded into iTunes first
