title: In Python, what is the fastest algorithm for removing duplicates from a list so that all elements are unique *while preserving order*?
body: <p>For example:</p>
  
  <pre><code>&gt;&gt;&gt; x = [1, 1, 2, 'a', 'a', 3]
  &gt;&gt;&gt; unique(x)
  [1, 2, 'a', 3]
  </code></pre>
  
  <p>Assume list elements are hashable.</p>
  
  <p><strong>Clarification:</strong> The result should keep the first duplicate in the list. For example, [1, 2, 3, 2, 3, 1] becomes [1, 2, 3].</p>