I never heard of fries cooked in duck oil, so I googled it. Seems that it's a common practice. Sounds like an excuse to charge 6 dollars for 25 cents worth of potatoes.
Potatoes fried in duck fat are one of the reasons life is worth living, imho. It's a classic French thing to do, actually. Whenever I make duck breasts, I fry potatoes (cubes, not sticks) in the rendered fat, and it is heavenly!!
Not to mention very easy to do. I highly recommend giving it a try, even if you're not making your own!!