Oh... Right. I suppose if iTunes exports it playlists as m3u files then when you import them it's going to also bring in the music again, causing the duplicates issue I mentioned.

One way around it might be to bring in the playlists first and then do a music drag from Explorer. Hopefully iTunes will ignore files that it already has indexed at that point. I know that for the most part I can get away with dragging a huge folder structure into iTunes and it will skip over or simply refresh the tracks it already has indexed, adding any it doesn't.

EDIT: I just did a test for you. Exporting the playlist and then later importing it after having added all your music will not duplicate the tracks. I tested with an export as plain text.