title: How to delete duplicate lines in a file without sorting it in Unix
body: <p>Is there a way to delete duplicate lines in a file in Unix?</p>
  <p>I can do it with <code>sort -u</code> and <code>uniq</code> commands, but I want to use <code>sed</code> or <code>awk</code>.</p>
  <p>Is that possible?</p>