If you're on Windows or Mac the Easy Duplicate File Finder has a good repletion, and costs $40 http://www.easyduplicatefinder.com/ I have no relationship to them.

If you search for "duplicate file finder" you'll find tons of free programs.

Do NOT install free software (unless it's open-source under a GNU license and has a good online reputation. Do your research BEFORE you install). DO NOT install the "free downloader utility" from download sites, as those usually contain some form of spyware/adware and you'll end up with pop-ups etc and those can be a conduit for spyware! "Free" usually comes with a payload (some form of spyware or malware) or it's a crippled/limited trial so that you'll have to pay the fee to get the use you need anyway. Go direct to the publishers website to download it, or skip it if they only offer the download via "download sites" (e.g. PC World, C-NET or Tucows).