Duties concern things that are voluntary. I do think that if you have a moral duty to bring me back the book you borrowed, that implies, roughly, that your doing so depends on your wanting to do so: if you want to bring me the book, you will. This is not the case if you are stuck at some airport due to a snowstorm, far away from me. This, however, is not the same as "ought" implying a metaphysical "can".