title: Pandas unstack problems: ValueError: Index contains duplicate entries, cannot reshape
body: <p>I am trying to unstack a multi-index with pandas and I am keep getting:</p>
  
  <pre><code>ValueError: Index contains duplicate entries, cannot reshape
  </code></pre>
  
  <p>Given a dataset with four columns:</p>
  
  <ul>
  <li>id (string)</li>
  <li>date (string)</li>
  <li>location (string)</li>
  <li>value (float)</li>
  </ul>
  
  <p>I first set a three-level multi-index:</p>
  
  <pre><code>In [37]: e.set_index(['id', 'date', 'location'], inplace=True)
  
  In [38]: e
  Out[38]: 
                                      value
  id           date       location       
  id1          2014-12-12 loc1        16.86
               2014-12-11 loc1        17.18
               2014-12-10 loc1        17.03
               2014-12-09 loc1        17.28
  </code></pre>
  
  <p>Then I try to unstack the location:</p>
  
  <pre><code>In [39]: e.unstack('location')
  ---------------------------------------------------------------------------
  ValueError                                Traceback (most recent call last)
  &lt;ipython-input-39-bc1e237a0ed7&gt; in &lt;module&gt;()
  ----&gt; 1 e.unstack('location')
  ...
  C:\Anaconda\envs\sandbox\lib\site-packages\pandas\core\reshape.pyc in _make_selectors(self)
      143 
      144         if mask.sum() &lt; len(self.index):
  --&gt; 145             raise ValueError('Index contains duplicate entries, '
      146                              'cannot reshape')
      147 
  
  ValueError: Index contains duplicate entries, cannot reshape
  </code></pre>
  
  <p>What is going on here?</p>