DupeList is a small program that will let you scan a folder on your computer for duplicate files. It will return a list of files that are duplicates, and can also delete the files in question for you.
It is recommended that you only scan folders that you organize yourself (such as My Pictures, for example). Often times application and data folders will purposefully contain duplicate files, so if you scan those types of folders, you may see a lot of duplicate files in your results.
DupeList is freeware.
Download DupeList (ClickOnce Installer)
Note: DupeList requires the .NET Framework 3.5 to be installed. If you do not have it, you will be prompted to install it.