duty vs should

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). 

should

noun
  • Something that ought to be the case as opposed to already being the case. 

verb
  • Will be likely to (become or do something); indicates a degree of possibility or probability that the stated thing will happen or be true in the future. 

  • Indicates that something is expected to have happened or to be the case now. 

  • Used to express a conditional outcome. 

  • With verbs such as 'see' or 'hear', usually in the second person, used to point out something remarkable in either a good or bad way. 

  • To make a statement of what ought to be true, as opposed to reality. 

  • Used to impart a tentative, conjectural or polite nuance. 

  • Used to express what the speaker would do in another person's situation, as a means of giving a suggestion or recommendation. 

  • Simple past tense of shall. 

  • In questions, asks what is correct, proper, desirable, etc. 

  • Used to issue an instruction (traditionally seen as carrying less force of authority than alternatives such as 'shall' or 'must'). 

  • Used to give advice or opinion that an action is, or would have been, beneficial or desirable. 

  • Used to form a variant of the present subjunctive, expressing a state or action that is hypothetical, potential, mandated, etc. 

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