Duty is work that you have to do for your job.

Your duties are tasks which you have to do because they are part of your job.

If you say that something is your duty, you believe that you ought to do it because it is your responsibility.

Duties are taxes which you pay to the government on goods that you buy.

If someone such as a policeman or a nurse is off duty, they are not working. If someone is on duty, they are working.