To remove duplicates in iTunes the best way is to use its “built-in duplicate finder”. Click File/Display Duplicates in iTunes and then delete the once you don’t need.
Besides it is pOSsible to delete files within your iTunes Library, but we’d like to warn you that it could harm the consistency of iTunes. Therefore we’re trying to protect users from doing that with warning messages, and not allow it by default.
If you just drop in your iTunes folder in the Startup window (or press + and select it). You’ll then see then all the duplicates, but you’ll get a warning message every time you try to select them (therefore automatic selection won’t select it as well). As a workaround for users who really know what they do we have an option in Preferences called “Disable item selection warnings”. That will help you out, but use it very carefully (it will still only work with auto select if you check the Allow checkbox).