Morality As a Limit
Duty is a moral obligation required by obedience to a higher authority. It is the belief that you must perform certain actions not because they provide benefits, but because they are demanded. The higher authority can be an ethical system, a king, your family, god, nature, or anyone or anything else that demands actions from you. There often is no reason for the demand, but this doesn't matter. Duty is an obligation without reason.
Duty is incompatible with a rational ethical system. It demands actions or prevents actions for no reason. It destroys the concept of an objective ethics by claiming a source of ethics not based in reality. It severs the link to reality, and divides morality from self-interest. Morality becomes a weapon against one's own life. The choice left is between being good, and living.