How can I delete duplicate songs

Easiest way is to change the view to SONGs and then sort the view by title of the song. That way all the duplicate songs will appear together.

To delete the actual songs you can either right click on the song and select DELETE SONG or you can highlight the song and hit the DELETE key on your keyboard.

Just be careful if you are deleting songs that are physically on your computer that you have a backup copy somewhere - incase you ever want to restore them.