title: why key is not unique in MapReduce function?
body: <p>when I research in MapReduce functionality in Hadoop, it was very common sample is "WordCounting", and also this image is very usual to show steps of Map action and reduce:
  <a href="https://wikis.nyu.edu/download/attachments/74681720/WordCount%20MapReduce%20Paradigm.PNG?version=1&amp;modificationDate=1462902481180&amp;api=v2" rel="nofollow noreferrer">https://wikis.nyu.edu/download/attachments/74681720/WordCount%20MapReduce%20Paradigm.PNG?version=1&amp;modificationDate=1462902481180&amp;api=v2</a>
  <a href="https://i.stack.imgur.com/199Q1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/199Q1.png" alt="enter image description here"></a></p>
  
  <p>why in mapping section keys are not unique? for example car.</p>