So Duquesne doesn't even know which collections agency handled my bill, (and I don't either since they turned off caller ID and I didn't have the foresight to ask them for their phone number since I assumed Duquesne would have it) so I can't even call them back, but instead wait until they call me, and dispute the charge. Moreover, the slight discrepancy between the amount originally owed to Duquesne ($83 and change) and the amount that showed up on my bank statement as paid ($89 and change) is completely right, as I thought it was --- there was a small extra charge for paying by credit card over the phone. The collections agency caller acted like I was completely insane that my payment was correct when I gave her a slightly differing amount. Gah.
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.