title: How to use sed to remove only double empty lines?
body: <p>I found this <a href="https://stackoverflow.com/questions/4651591/howto-use-sed-to-remove-only-triple-empty-lines">question and answer</a> on how to remove triple empty lines. However, I need the same only for double empty lines. Ie. all double blank lines should be deleted completely, but single blank lines should be kept.</p>
  
  <p>I know a bit of sed, but the proposed command for removing triple blank lines is over my head:</p>
  
  <p><code>sed '1N;N;/^\n\n$/d;P;D'</code></p>