Franbern, I have been thinking about your post all day. I think the word duty has several quite nuanced meanings.
A doctor, nurse, or any other person in work has their duty to their employer, who is paying them,. But that duty is limited to the conditions of employment and hours of work. duty, in the Queens sinse would be when they decide to work longer hours or call in when off duty because of their concern about a patient - and remember, medical staff like the rest of the world can be good bad and indifferent, and some will skive and ignore patients, or leave them waiting for care, just as some will go to great lengths to care and there are enough compensation payments and hospital scandals to show that not every problem can be blamed on inadequste staffing.
The same applies to the Queen, there are certain duties, in the term of things she is expected to do, and is paid for, but her determination to continue to do things well beyond the stage when she could reasonably pack it in and retire to Windsor and only be seen on postage stamps.
can reasonably be defined as committed to doing her duty well beyond what she is paid for - and still doing it very well at 96.