title: Swift Dictionary: How to merge values in dictionary contains similar keys
body: <p>I have a dictionary as follows:</p>
  
  <pre><code>var dict1 = [2019 : [J,F,M], 2019 : [J,S,O],2018 : [J,F,M], 2018 : [J,S,O]]
  </code></pre>
  
  <p>expected output:</p>
  
  <pre><code>var dict1 = [2019 : [J,F,M,S,O], 2018 : [J,F,M,S,O]]
  </code></pre>
  
  <p>I want to merge the unique values in a dictionary contains duplicate keys.</p>