BUG: Indexing with duplicate columns
Currently failing test case:

``` python
df = DataFrame([[1,2,'foo','bar']], columns=['a','a','a','a'])
s = Series([1, 2, 'foo', 'bar'], index=['a', 'a', 'a', 'a'], name=0)
assert_series_equal(df.irow(0), s)
```

Here's what's going on right now.

``` python
In [3]: from pandas import *
In [4]: df = DataFrame([[1,2,'foo','bar']], columns=['a','a','a','a'])
In [5]: df
Out[5]:
   a  a    a    a
0  1  2  foo  bar

In [6]: df.irow(0)
Out[6]:
a    bar
a    bar
a    bar
a    bar
Name: 0, dtype: object

In [7]: df.icol(0)
Out[7]:
0    1
Name: a, dtype: int64

In [8]: df.columns
Out[8]: Index([u'a', u'a', u'a', u'a'], dtype=object)
```
