title: Using rolling with datetime index and duplicate timestamps
body: <p>I have a pandas dataframe <code>df</code> that goes like this</p>
  <pre><code>index     data
  midnight  1
  1am       1
  2am       1
  2am       1
  </code></pre>
  <p>I want to count the number of lines in the last 90 minutes excl, to do so, I do
  <code>df.rolling(&quot;90min&quot;, closed=&quot;left&quot;).fillna(0)</code></p>
  <p>Result is</p>
  <pre><code>index     data
  midnight  0
  1am       1
  2am       1
  2am       2
  </code></pre>
  <p>Considering <code>2am</code> is a duplicate index, I would like the last value to be <code>1</code> instead of <code>2</code>. Any idea ?</p>