Free and – unlike the also free [WayBack] dupeGuru – finds duplicate files – hasn’t crashed on my system yet or fails to add folders:
Read reviews, compare customer ratings, see screenshots, and learn more about Duplicate File Finder for files & folders. Download Duplicate File Finder for files & folders for Mac OS X 10.9 or later and enjoy it on your Mac.
[WayBack] Duplicate File Finder for files & folders on the Mac App Store
–jeroen