title: Python argparse: Force a list item to be unique
body: <p>Being able to validate the list items using <code>choices=servers</code> below is nice. </p>
  
  <pre><code>servers = [ "ApaServer", "BananServer", "GulServer", "SolServer", "RymdServer", "SkeppServer", "HavsServer", "SovServer" ]
  parser = argparse.ArgumentParser()
  parser.add_argument('-o', '--only', nargs='*', choices=servers, help='Space separated list of case sensitive server names to process')
  </code></pre>
  
  <p>Is it possible to force an item in the list to be unique, so that no duplicates are allowed?</p>