Does your hard drive appear to be full of documents, photos and music? It may not be as full as it looks thanks to duplicate files.
Duplicate files are especially a problem for large digital photo or music collections. Fortunately, sorting and comparing files happens to be a specialty of computers; you just need the right software.
CloneSpy is a free program that finds duplicate files, even if they have different file names. It can delete redundant files automatically or move them to a separate folder for you to peruse.
For safety, I would avoid automatic deletion. You don't want to accidentally delete important files.