jEmplode already does this, but I don't know how perfected this functionality is with regards to when you modify fields on the player, or modify the file on your PC. In other words, I'm not sure what it uses to determine uniqueness/duplicates. I think I remember running into an issue or two with it.

I thought this was suggested for Emplode, and I think it might actually be there in some form... But I haven't been following too closely. What I do now is manually drop my new MP3's into the PC's directory structure and immediately to Emplode. It's not convenient and a more robust sync mechanism would be nice... I think jEmplode's probably works better than I'm giving it credit for, I just wasn't mentally strong enough to use it exclusively in the case that something went wrong.