title: Duplicate each line in VI
body: <p>I have a file with these lines:</p>
  
  <pre><code>aa
  bb
  cc
  dd
  </code></pre>
  
  <p>I want to convert this into:</p>
  
  <pre><code>aa
  aa
  bb
  bb
  cc
  cc
  dd
  dd
  </code></pre>
  
  <p>Is this possible in VI?</p>