Pandas error

Traceback (most recent call last):
  File "./denormalize.py", line 185, in <module>
    df_join = pd.concat([d.set_index('USER_ID') for d in df_arr], axis=1, keys=sql_tables)
  File "/usr/local/lib/python2.7/dist-packages/pandas/tools/merge.py", line 845, in concat
    copy=copy)
  File "/usr/local/lib/python2.7/dist-packages/pandas/tools/merge.py", line 984, in __init__
    self.new_axes = self._get_new_axes()
  File "/usr/local/lib/python2.7/dist-packages/pandas/tools/merge.py", line 1060, in _get_new_axes
    new_axes[i] = self._get_comb_axis(i)
  File "/usr/local/lib/python2.7/dist-packages/pandas/tools/merge.py", line 1086, in _get_comb_axis
    return _get_combined_index(all_indexes, intersect=self.intersect)
  File "/usr/local/lib/python2.7/dist-packages/pandas/indexes/api.py", line 37, in _get_combined_index
    union = _union_indexes(indexes)
  File "/usr/local/lib/python2.7/dist-packages/pandas/indexes/api.py", line 71, in _union_indexes
    result = result.union(other)
  File "/usr/local/lib/python2.7/dist-packages/pandas/indexes/base.py", line 1737, in union
    indexer = self.get_indexer(other)
  File "/usr/local/lib/python2.7/dist-packages/pandas/indexes/base.py", line 2082, in get_indexer
    raise InvalidIndexError('Reindexing only valid with uniquely'
pandas.indexes.base.InvalidIndexError: Reindexing only valid with uniquely valued Index objects