Is it duplicating entries in your iTunes library, or duplicating files on your hard drive? If the latter, uncheck the following boxes:
If it's the former, in iTunes click File > Display Duplicates and make sure they're true duplicates. iTunes matches Artist and song name to determine duplicates, but they may come from different albums, one may be a live, acoustic or extended version, they may have different bit rates or duration. In some cases, it's preferable to have duplicates, such as wanting to keep albums intact. No app can make those determinations. It's safer to manually determine what is a true duplicate that you want to get rid of.