title: find row before rows with same values in pandas dataframe
body: <p>I have a dataframe like this:</p>
  
  <pre><code>  peak-date  
  0    17 Jan  
  1    17 Jan  
  2    31 Mar  
  3    30 Apr  
  4    31 May  
  5    26 Jun  
  6    26 Jun  
  </code></pre>
  
  <p>I want to find the row before the row when peak-date values become the same. In this case that would be the row with <code>peak-date</code> of <code>31 May</code>. I can do this fo r numeric values using df['peak-date'].diff(), but how do I do it here?</p>