Adding songs to a playlist on iPod Nano- playlist is duplicated when syncing with iTunes

When I use my iPod Nano to add songs to a playlist that I sync with iTunes, the playlists do not sync properly. When I sync, a duplicate version of the playlist is created in iTunes from the Nano instead of adding the new songs to the original playlist. For example, I have a playlist called "Blues" in iTunes that I sync with my iPod Nano. When playing music from my Nano, I come across a song that I want to add to the playlist, so I add it using the Nano. When I sync with iTunes later, instead of adding that song to the "Blues" playlist, iTunes creates a playlist called "Blues 1", which has all of the songs from the "Blues" playlist plus the song that I added on the Nano.

The playlists that I am trying to add songs to and sync are in a playlist folder (which shouldn't matter, but this might help with troubleshooting).