reindexing non-unique dataframes
> > > df3 = DataFrame({'a':[1]})
> > > df4 = DataFrame({'a':[1,2]})
> > > df3
> > >    a
> > > 0  1
> > > df4
> > >    a
> > > 0  1
> > > 1  2
> > > df4.index = [1,1]
> > > df4.reindex(df3.index)
> > > Traceback (most recent call last):
> > >   File "<stdin>", line 1, in <module>
> > >   File "/hostname/home/chik/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/pandas/core/frame.py", line 2347, in reindex
> > >     'column labels')
> > >   File "/hostname/home/chik/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/pandas/core/frame.py", line 2423, in _reindex_index
> > >     clidx, cridx = None, None
> > >   File "/hostname/home/chik/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/pandas/core/index.py", line 828, in reindex
> > >     return com._ensure_platform_int(indexer)
> > >   File "/hostname/home/chik/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/pandas/core/index.py", line 749, in get_indexer
> > >     def set_value(self, arr, key, value):
> > > Exception: Reindexing only valid with uniquely valued Index objects

Why is this not possible?
