title: Combining two lists and removing duplicates, without removing duplicates in original list
body: <p>I have two lists that i need to combine where the second list has any duplicates of the first list ignored. .. A bit hard to explain, so let me show an example of what the code looks like, and what i want as a result.</p>
  
  <pre><code>first_list = [1, 2, 2, 5]
  
  second_list = [2, 5, 7, 9]
  
  # The result of combining the two lists should result in this list:
  resulting_list = [1, 2, 2, 5, 7, 9]
  </code></pre>
  
  <p>You'll notice that the result has the first list, <em>including</em> its two "2" values, but the fact that second_list also has an additional 2 and 5 value is not added to the first list.</p>
  
  <p>Normally for something like this i would use sets, but a set on first_list would purge the duplicate values it already has. So i'm simply wondering what the best/fastest way to achieve this desired combination.</p>
  
  <p>Thanks.</p>