title: Repeat each line using sed
body: <p>I want to duplicate each line of the input using sed. I.e. when the input looks like:</p>
  
  <pre><code>first
  secod
  third
  fourth
  ...
  </code></pre>
  
  <p>I want the output to look like:</p>
  
  <pre><code>first first
  second second
  third third
  fourth fourth
  ... ...
  </code></pre>
  
  <p>and so on.</p>
  
  <p>If this is impossible using sed, then what do you recommend? I prefer the most simple solution.</p>