Johnas33,
I tried this out a few times just to be sure I got it right, and I didn't get any duplicates of the same song, even though I have quite a few songs of the same name, but different artists. HOWEVER, if you haven't searched for duplicates there may be songs by the same artist BUT from a different album included in the list. I tend to cull my library of duplicates (a nifty way is by adding a smartlist from the playlists tree - right click, choose make new smartlist, then expand the smartlist rules menu and choose possible duplicates).
making a large playlist shouldn't take very long - I have over 13,800 tunes on my Zen, but I made a smartlist (Audio- All) and transfer it and any other playlist I made and want on the Zen when I do a synchronization (I just check the playlists that I want and leave the others blank) and check the sync playlists box. This usually doesn't take very long and it updates the playlists and the Zen with any changes I might have made in MC. hope this helps. Bet