Why do I get duplicate playlists on my iPod nano?

If I add a song to a playlist on my Nano and then sync my iPod with my computer, it creates a duplicate playlist. This seems silly, a duplicate playlist just because I added 1 song. Why doesn't sync just update the playlist in ITunes with the new song from the playlist on my Nano? I've been poking around the settings and I can't figure it out. Any ideas?