If you are the type of person who’d rather do things the traditional way, and not hassle himself in downloading apps that can potentially be dangerous, the best way to Delete iTunes Duplicates is to just manually do it. You can just direct yourself to the display duplicates option (which can be found at File menu), and delete one by one. You may use filter options to help you discern.
However, there are available freeware installers that you can download to make your life easier. If you fear risks for your computer, look at reviews from sites and recommendations from other people. These can give you validations if they are safe.