In summary, Duquesne told me the only thing that I can do is ask formally to dispute the charge when the collections people call me again, and they will call Duquesne, and find out I have a balance of $0. Why didn't they just do that when I was, well, DISPUTING THAT I OWED THE MONEY, is beyond me.
MANY SWEAR WORDS IN CAPITAL LETTERS ARGH.