title: How to remove duplicate lines from a file
body: <p>I have a tool that generates tests and predicts the output.  The idea is that if I have a failure I can compare the prediction to the actual output and see where they diverged.  The problem is the actual output contains some lines twice, which confuses <code>diff</code>.  I want to remove the duplicates, so that I can compare them easily.  Basically, something like <code>sort -u</code> but without the sorting.</p>
  
  <p>Is there any unix command line tool that can do this?</p>