title: Union of two lists in Python
body: <p>I have two lists say:</p>
  
  <p><code>a = [1, 2, 2, 2, 3]</code></p>
  
  <p><code>b = [2, 5, 6]</code></p>
  
  <p>After doing a union, I should get something like this (don't mind the order):</p>
  
  <p><code>c = [1, 2, 2, 2, 3, 5, 6]</code></p>
  
  <p>The final list should contain common elements only once and rest of elements (from both lists) should be copied as they are. Sets can't be used for this as they remove multiple occurrence of an element from the list, which isn't a union. What is a Pythonic way to do so?</p>