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To remove the duplicates from your iTunes library can be done two ways. One is to remove them one at a time, which takes quite a bit of time with a library the size of yours.
The other is to use a third party utility, but none of these are free.
There is one that will give you full functionality for 30 days - you can use this one to remove your duplicates and try the software, then uninstall it later if you decide not to keep it.
The name of the software is Dupe Eliminator. It is specifically made for iTunes and will remove your duplicates. Here is the download link:
Here is another that is a paid version - the price of this one is only $7.99 compared to the $34.95 of the other:
iTunes Duplicate Remover