You can delete duplicate music and video files on iTunes if you are using version 8 or higher. The iTunes application will scan its music library and show you all the files it believes are the same. Deleting files is a two-step process in iTunes. The first step deletes the song from appearing in your iTunes library. If you want to delete the file from your hard drive, you must also also select "Move to Trash" during the second step. You can also set your iTunes preferences so that iTunes will prevent you from adding duplicate files in the future.
Launch iTunes on your computer. Click "Music" in the "Library" section of the left menu to see all your music files.
Click the "File" menu, and then select "Show Duplicates" from the drop-down list. After a few seconds, iTunes will display only duplicate files.
Click any song and press the "Delete" key. A dialog box will open, asking you to confirm that you want to delete the duplicate.
Click the "Remove" button. This removes the song's listing from iTunes. A second dialog box will ask you if you want to move song file to the Trash, or keep it in the iTunes Music folder on your hard drive.
Click "Move to Trash."
Click "Movies" in the "Library" section of the left menu. Repeat these steps to delete duplicate videos.
Preventing Future Duplicates
Launch iTunes and click the "Edit" menu. Select "Preferences." Click "Advanced."
Select "Keep iTunes Music Folder Organized" and "Copy Files to iTunes Music Folder When Adding to Library" by clicking the boxes beside them so there is a check mark in the box.
Click "OK" to save your selections and close the Preferences window. The iTunes application will now prevent duplicate files from being added to its music folders on your hard drive. Instead, it will show you where the song already is in your iTunes Library.