Are these physical dupes, with more than 1 copy of the file in either the same folder or two different locations? Or are they logical dupes, multiple entries in the library to the same physical file? Worst scenario is that there is a mix.
If everything is one type or the other then:
- Select the List view
- Hold down Option, then click iTunes > Show Exact Duplicates (Hold Shift, File > Show Exact Duplicates on a PC)
- Turn on the Date Added column if not already present
- Sort the listing on Date Added
- Scroll down the list to the point when all the duplicates were created
- Select all the duplicates to delete and delete them
- For physical dupes allow the files to be sent to the trash, for logical dupes keep the files
If there is a mix, or they've built up over time, then it probably best to use a third party tool such as Dupin or Dupin Lite.
tt2