title: removing repeating lines from the file
body: <p>which is the simplest way to remove:</p>
  
  <ol>
  <li>appearances of the single line series in a row</li>
  <li>appearances of the double line series in a row</li>
  <li>appearances of the triple line series in a row</li>
  </ol>
  
  <p>according to example? Is it possible to solve this using Linux script?
  thank you
  P.</p>
  
  <p>Input file:</p>
  
  <pre><code>text1
  text2
  text3
  text1
  text2
  text3
  text1
  text2
  text3
  text4
  text5
  text4
  text5
  text1
  text1
  text4
  text5
  text4
  text5
  text4
  text5
  text4
  text5
  text4
  text5
  text1
  text3
  </code></pre>
  
  <p>Output file:</p>
  
  <pre><code>text1
  text2
  text3
  text4
  text5
  text1
  text4
  text5
  text1
  text3
  </code></pre>