I see this is your first post on MacRumors Forums and would like to be the first to welcome you.
When making a playlist, iTunes makes a path to the song. It doesn't duplicate it because that would use hard drive space on your computer and iPod Touch. When you make a playlist in iTunes, you can drag the songs in the playlist or secondary click and hover over "Add To Playlist" and select the playlist you want to add it to. You only need one copy of the song to put it into as many playlist that you want. Post here or Private Message (PM) me with any other questions you have.