Look up "Senuti" on Google and download it. It works with both Windows and Mac formatted iPods and allows you to download any music/video file on an iPod to your Mac. Also allows you to download playlists from an iPod to a Mac.
If you set the folder that Senuti downloads your files into to the ~/User/Music/iTunes/iTunes Music then you won't get duplicates of your music/video files on your Mac.
It's a great little free program. Essential really.