Before performing any steps, make sure your iTunes is updated to the latest version; for it, go to 'Help' and select 'Check for Update'.
If you're already using the new version and want to delete the duplicate song, go to 'View' press on 'Show Duplicate Items'.
Now appear all the music you have twice; to delete an item, click on it with the right mouse button and press 'Delete'.
Anyway, if you press the 'Command' key on Mac or 'Control' in Windows, you can go selecting multiple songs at once, so linters once.