title: ValueError: Index contains duplicate entries, cannot reshape
body: <p>I'm trying to reshape my pd dataframe with the following function:</p>
  
  <pre><code> ar = ar.pivot(index='Received', columns='Merch Ref', values='acceptance_rate')
  </code></pre>
  
  <p>The dataset looks like:</p>
  
  <pre><code>     Merch Ref            Received  acceptance_rate
  0           SF 2014-08-28 15:38:00                0
  1           SF 2014-08-28 15:44:00                0
  2           SF 2014-08-28 16:04:00                0
  3           WF 2014-08-28 16:05:00                0
  4           WF 2014-08-28 16:07:00                0
  5           SF 2014-08-28 16:34:00                0
  6           SF 2014-08-28 16:55:00                0
  7           BF 2014-08-28 17:59:00                0
  8           BF 2014-08-29 15:05:00                0
  9           SF 2014-08-29 21:25:00                0
  10          SF 2014-08-30 10:29:00                0
  ...
  </code></pre>
  
  <p>What I'd like to obtain is:</p>
  
  <pre><code>                      SF WF BF 
  2014-08-28 15:38:00    0  1  0
  2014-08-28 15:44:00    0  1  0
  2014-08-28 16:04:00    0  0  1
  2014-08-28 16:05:00    1  1  0
  2014-08-28 16:07:00    0  0  1
  2014-08-28 16:34:00    1  1  0
  2014-08-28 16:55:00    1  1  0
  2014-08-28 17:59:00    0  1  0
  2014-08-29 15:05:00    0  0  1
  2014-08-29 21:25:00    0  0  1 
  2014-08-30 10:29:00    0  1  0
  </code></pre>
  
  <p>However, I get the error: </p>
  
  <pre><code> ValueError: Index contains duplicate entries, cannot reshape
  </code></pre>
  
  <p>This is because i have some orders placed at the same time. Is there a way to sum/aggregate these orders ? </p>