title: Remove duplicates based on the content of two columns not the order
body: <p>I have a correlation matrix that i melted into a dataframe so now i have the following for example: </p>
  
  <pre><code>First      Second       Value
  A          B            0.5
  B          A            0.5
  A          C            0.2 
  </code></pre>
  
  <p>i want to delete only one of the first two rows. What would be the way to do it?</p>