title: find duplicates in two columns and extract them to a new column
body: <p>I have two columns with different lengths contains numbers like the following:</p>
  
  <pre><code>column 1  column 2
  
  1          8
  2          9
  20         1
  4          2
  5          20 
  6         
  </code></pre>
  
  <p>what I want to do is find the duplicates in the two columns and place them in a new column like the following:</p>
  
  <pre><code>column 1  column 2  column 3
  
  1          8          1
  2          9          2
  20         1         20
  4          2
  5          20 
  6        
  </code></pre>
  
  <p>so is there any way that I can get this result in excel?</p>
  
  <p>Thanks in advance!</p>