In preparation for moving a load of data in Google Drive I needed to de-dupe a bunch of files.
Duplicate Files Searcher from http://duplicatefilessearcher.net/ is a good app. Yes it runs under Java, but does the job, and even better, free.
Add your Directories to search, flick to 'Searching Options', decide if you want to compare by file name as well as content (uses hash functions) and off you go.
Pretty quick to search my Dropbox folder with ~2500 files. Turned out I had about 800 dupes, and saved me 500MB of space.