0.13.1: strange join error
I am puzzled by the following

```
    logger.debug('Indexes are identical: {}'.format(data_frame.index is categories.index))
    logger.debug('Index is unique: {}'.format(data_frame.index.is_unique))
    logger.debug('All columns are in data_frame: {}'.format(all(map(lambda x: x in data_frame, columns_regression))))
    df_regression = data_frame[columns_regression].join(categories)
```

```
2014-06-23 21:02:56,125 DEBUG    pmt.portfolio_analytics.attribution.AlphaData - Indexes are identical: True
2014-06-23 21:02:56,141 DEBUG    pmt.portfolio_analytics.attribution.AlphaData - Index is unique: True
2014-06-23 21:02:56,142 DEBUG    pmt.portfolio_analytics.attribution.AlphaData - All columns are in data_frame: True
Traceback (most recent call last):
  File "C:/Users/BKovtunenko/Perforce/bos-perforce01_1666/bkovtunenko_BKOVTUNENKO-D7/aam/branches/boris/pmt/python/pmt/portfolio_analytics/attribution/AlphaData.py", line 1782, in <module>
    thresholds_mkt_cap=thresh_mkt_cap)
  File "C:/Users/BKovtunenko/Perforce/bos-perforce01_1666/bkovtunenko_BKOVTUNENKO-D7/aam/branches/boris/pmt/python/pmt/portfolio_analytics/attribution/AlphaData.py", line 1640, in save_regression_results
    regression_type=regresion_type)
  File "C:/Users/BKovtunenko/Perforce/bos-perforce01_1666/bkovtunenko_BKOVTUNENKO-D7/aam/branches/boris/pmt/python/pmt/portfolio_analytics/attribution/AlphaData.py", line 1489, in save_regression_decomposition
    df_regression = data_frame[columns_regression].join(categories)
  File "C:\Python27\lib\site-packages\pandas\core\frame.py", line 3580, in join
    rsuffix=rsuffix, sort=sort)
  File "C:\Python27\lib\site-packages\pandas\core\frame.py", line 3594, in _join_compat
    suffixes=(lsuffix, rsuffix), sort=sort)
  File "C:\Python27\lib\site-packages\pandas\tools\merge.py", line 40, in merge
    return op.get_result()
  File "C:\Python27\lib\site-packages\pandas\tools\merge.py", line 189, in get_result
    ldata, rdata = self._get_merge_data()
  File "C:\Python27\lib\site-packages\pandas\tools\merge.py", line 284, in _get_merge_data
    copydata=False)
  File "C:\Python27\lib\site-packages\pandas\core\internals.py", line 3439, in _maybe_rename_join
    to_rename = self.items.intersection(other.items)
  File "C:\Python27\lib\site-packages\pandas\core\index.py", line 962, in intersection
    indexer = self.get_indexer(other.values)
  File "C:\Python27\lib\site-packages\pandas\core\index.py", line 1120, in get_indexer
    raise InvalidIndexError('Reindexing only valid with uniquely'
pandas.core.index.InvalidIndexError: Reindexing only valid with uniquely valued Index objects

Process finished with exit code 1
```
