reindexing non-unique dataframes

[Aico]

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 "", line 1, in
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?