title: How to keep rows without duplicate in R
body: <p>I have a data frame with many duplicated rows:</p>
  
  <pre><code>1    3
  1    2
  2    5
  2    4
  2    5
  3    7
  3    7
  3    7
  3    7
  3    7
  3    8
  3    7
  3    7
  4    2 
  4    5 
  4    2
  </code></pre>
  
  <p>I want to keep just rows that repeat once in data.frame:</p>
  
  <pre><code>1    3
  1    2
  2    4
  3    8
  4    5 
  </code></pre>
  
  <p>Would you please guide me?</p>