title: pandas crashes on repeated DataFrame.reset_index()
body: <p>Very weird bug here: I'm using pandas to merge several dataframes.  As part of the merge, I have to call reset_index several times.  But when I do, it crashes unexpectedly on the second or third use of reset_index.</p>
  
  <p>Here's minimal code to reproduce the error:</p>
  
  <pre><code>import pandas
  A = pandas.DataFrame({
      'val' :  ['aaaaa', 'acaca', 'ddddd', 'zzzzz'],
      'extra' : range(10,14),
  })
  A = A.reset_index()
  A = A.reset_index()
  A = A.reset_index()
  </code></pre>
  
  <p>Here's the relevant part of the traceback:</p>
  
  <pre><code>....
      A = A.reset_index()
    File "/usr/local/lib/python2.7/dist-packages/pandas/core/frame.py", line 2393, in reset_index
      new_obj.insert(0, name, _maybe_cast(self.index.values))
    File "/usr/local/lib/python2.7/dist-packages/pandas/core/frame.py", line 1787, in insert
      self._data.insert(loc, column, value)
    File "/usr/local/lib/python2.7/dist-packages/pandas/core/internals.py", line 893, in insert
      raise Exception('cannot insert %s, already exists' % item)
  Exception: cannot insert level_0, already exists
  </code></pre>
  
  <p>Any idea what's going wrong here?  How do I work around it?</p>