Selecting multiple columns from DataFrame with duplicate column labels failure.
``` python
In [25]: df = pandas.DataFrame(np.random.randn(4,4), columns=list('AABC'))

In [26]: df
Out[26]: 
          A         A         B         C
0 -0.174905  0.332522  1.134984 -0.201270
1  1.730445  0.382556 -0.607761  1.221815
2  0.513049  0.196231 -1.746732 -0.252282
3 -0.297577 -1.000121 -0.090442 -2.129467

In [27]: df.ix[:,['A', 'B']]
Out[27]: 
          A         A         B
0 -0.174905  0.332522  1.134984
1  1.730445  0.382556 -0.607761
2  0.513049  0.196231 -1.746732
3 -0.297577 -1.000121 -0.090442

In [28]: df[['A', 'B']]
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
...
Exception: Reindexing only valid with uniquely valued Index objects

In [29]: df[['B', 'C']]
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
...
Exception: Reindexing only valid with uniquely valued Index objects

```
