title: Concatenate rows in a dataframe
body: <p>I have a dataframe structured like below:</p>
  
  <pre><code>Column A  Column B
  
  1          A  
  1          B  
  1          C  
  1          D  
  2          B  
  2          C  
  2          D  
  2          E 
  </code></pre>
  
  <p>I want to concatenate all the rows belonging to a specific value in column A.</p>
  
  <p>I want the final output to look something like this:</p>
  
  <pre><code>Column A Column B Column C  
  1        A        ABCD    
  1        B        ABCD  
  1        C        ABCD  
  1        D        ABCD  
  2        B        BCDE  
  2        C        BCDE  
  2        D        BCDE  
  2        E        BCDE   
  </code></pre>
  
  <p>How would I perform this operation in R/ Python?</p>
  
  <p>Thanks </p>