duty vs fee

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). 

fee

noun
  • A right to the use of a superior's land as a stipend for certain services to be performed, typically military service. 

  • An additional monetary payment charged for a service or good that is minor compared to the underlying cost. 

  • Synonym of fief: the land so held. 

  • An inheritable estate in land, whether absolute and without limitation to potential heirs (fee simple) or with limitations to particular kinds of heirs (fee tail). 

  • An inheritable estate in land held of a feudal lord on condition of performance of certain services, typically military service. 

verb
  • To reward for services performed, or to be performed; to recompense; to hire or keep in hire; hence, to bribe. 

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