title: Duplicating pandas dataframe vertically
body: <p>I have the foll. dataframe:</p>
  
  <pre><code>    Month  Day   season
  0       4   15  current
  1       4   16  current
  2       4   17  current
  3       4   18  current
  4       4   19  current
  5       4   20  current
  </code></pre>
  
  <p>I would like to duplicate it like so:</p>
  
  <pre><code>    Month  Day   season
  0       4   15  current
  1       4   16  current
  2       4   17  current
  3       4   18  current
  4       4   19  current
  5       4   20  current
  6       4   15  past
  7       4   16  past
  8       4   17  past
  9       4   18  past
  10       4   19  past
  11       4   20  past
  </code></pre>
  
  <p>I can duplicate it using:</p>
  
  <pre><code> df.append([df]*2,ignore_index=True)
  </code></pre>
  
  <p>However, how do I duplicate so that the <code>season</code> column has <code>past</code> as the duplicated values instead of <code>current</code></p>