Indexing issue for dataframes with duplicate indexes  
Consider:

df1 = DataFrame(np.arange(9.).reshape((3, 3)), columns=list('bcd'),
index=['a', 'a', 'c'])

Then, 
df1.loc[['a','c'],['b','c']] results in a key error

while,
df1.loc[['a','c'],:].loc[:,['b','c']]  produces the intended result.
