duty vs free will

duty

noun
  • That which one is morally or legally obligated to do. 

  • The state of being at work and responsible for or doing a particular task. 

  • A tax placed on imports or exports; a tariff. 

  • The efficiency of an engine, especially a steam pumping engine, as measured by work done by a certain quantity of fuel; usually, the number of pounds of water lifted one foot by one bushel of coal (94 lbs. old standard), or by 1 cwt. (112 lbs., England, or 100 lbs., United States). 

free will

noun
  • The ability to choose one's actions, or determine what reasons are acceptable motivation for actions, without predestination, fate etc. 

  • A person's natural inclination; unforced choice 

How often have the words duty and free will occurred in a corpus of books? (source: Google Ngram Viewer )