title: Filter a list to only leave objects that occur once
body: <p>I would like to filter this list,</p>
  <pre><code>[0, 1, 1, 2, 2]
  </code></pre>
  <p>to only leave</p>
  <pre><code>[0]
  </code></pre>
  <p>I'm struggling to do it in a 'pythonic' way. Is it possible without nested loops?</p>