Much like love, duty is tricky in The Plague, especially when you run into the notion of conflicting or competing obligations. Which is higher, the duty to the state, or to a marriage? To an individual’s suffering, or to the good of society as a whole? Characters deal with emotional, legal, moral, ethical, and religious duty in The Plague. Most interesting, however, is the distinction between common decency and heroism. The Plague argues that to put oneself at risk for the greater good doesn’t deserve medals or fanfare – it is simple a case of a man doing his job.
Rieux does his duty as a person, not as a doctor. When he says it is common decency to do one’s job, he’s referring to man’s common job as a citizen of the world, not his own job as a medical professional.
Grand is more of a hero than Rieux because the doctor does his job as a doctor, whereas Grand does his duty as a human being.