So I replied again, and explained the bug, and how you cannot obtain Lady Duff since Mexican Duffman is owned. So the 200 donuts to buy her with, while nice (even though I have thousands and don't need them), certainly isn't a solution to the problem.
And I got this back:
"I wish if I could add the Lady Duff to your account, however there is no such option available with us.
However the best what we can do is we have added 200 donuts to your account. I would really appreciate your patience and understanding."
So I guess that's it then! I'll have a quest in my list forever with no way to resolve. Hey, at least I got another 200 donuts I don't need!