当Series或DataFrame存在重复索引时，使用reindex()函数会抛出上述错误:

a = pd.Series([1,2,3,4,5,6],index=['a','b','c','d','e','a'])
print(a)

        a    1
        b    2
        c    3
        d    4
        e    5
        a    6

a.reindex(['b','c','e','b'])

报错：

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python\lib\site-packages\pandas\core\series.py", line 3325, in reindex
    return super(Series, self).reindex(index=index, **kwargs)
  File "C:\Python\lib\site-packages\pandas\core\generic.py", line 3689, in reindex
    fill_value, copy).__finalize__(self)
  File "C:\Python\lib\site-packages\pandas\core\generic.py", line 3707, in _reindex_axes
    copy=copy, allow_dups=False)
  File "C:\Python\lib\site-packages\pandas\core\generic.py", line 3810, in _reindex_with_indexers
    copy=copy)
  File "C:\Python\lib\site-packages\pandas\core\internals.py", line 4414, in reindex_indexer
    self.axes[axis]._can_reindex(indexer)
  File "C:\Python\lib\site-packages\pandas\core\indexes\base.py", line 3576, in _can_reindex
    raise ValueError("cannot reindex from a duplicate axis")
ValueError: cannot reindex from a duplicate axis

可以通过index.duplicated()函数找出重复索引：

a.index.duplicated()

array([False, False, False, False, False,  True])