There are a bunch of apps (free, shareware & commercial) that will offer to find and delete all of your duplicate files, but in my experience some of them go too far. Still, there are duplicates that can really eat up drive space and are easy to find. When I first started using iTunes, I decided to let it copy all files to the iTunes library. It was a way to make easy backups. Of course, back then my only music files were either downloaded from the iTunes store or iTunes imported them from actual CDs.
Now, I probably buy as much music from Amazon as I do from Apple. I price check, and if the Amazon price is better… — hey, I like to save money. But, I discovered that the Amazon downloader doesn’t just add the files to iTunes. It creates its own folder. When I thought to check it last week, the volume total of that one folder was nearly 10 GB of music that was also in my iTunes library folder. Add to that video files that were converted for iPad, and that was almost another 100 GB of duplicates. It adds up.
So, if you are wondering why all of that drive space has disappeared, check for extra folders (like Amazon MP3). And thin out those downloaded iPhone/iPad apps that you never use, too. You might be surprised at the size of some of them.