A good solution to detect and delete the duplicate files from a pc is to install a free and small software, named Easycleaner. I am using it for a while, successfully. The most powerful function you can get using Easycleaner is cleaning the registry. Other things you can do with: delete unnecessarily files and internet cookie files, manage programs starting at startup, add/remove programs (clean install and uninstall!), clear invalid shortcuts and history. Also, Easycleaner give you interesting informations about disk usage. Personally I used it mostly to check for duplicates files.

To get it, go to http://personal.inet.fi/business/toniarts/ecleane.htm and then download files/EClea2_0.zip
You don’t need to install anything, just double-click on exe file and you have open the program. Go to Options / Duplicate files and be sure you have checked, in require section, all four boxes.

My advice:
Be aware to use this software because if you go wrong is possible to lose important files. Be careful which files are you deleting!