How to delete multiple files from your iTunes library.
From Inside iTunes:
If you have a large song library, duplicate copies are inevitable. To trim down to only what you want, use the Display Duplicates command in the File menu, then delete the songs you want to get rid of (you'll have the option to remove them from iCloud as well, and to put them in the trash on your computer).
One user suggests the following thread (a snippet below, follow link for more):
From Apple Support:
You may sometimes see an item displayed more than once in your iTunes library. To find and remove all items that show up more than once in your library, follow the steps below. (follow link here or above)
A third-party app for sale is Tunesweeper.
"Tune Sweeper's unique design makes it simple to browse and safely remove duplicate and missing tracks in your digital music collection. Tune Sweeper will also quickly find tracks on your hard disk that are not currently in your iTunes library."