BUG: cannot insert/delete to non-unique columns
This currently breaks

```
In [1]: df = DataFrame([[1,1,1,5],[1,1,2,5],[2,1,3,5]],columns=['foo','bar','foo','hello'])

In [2]: df['string'] = 'bar'

In [3]: df
IndexError: ('index out of bounds', u'occurred at index hello')
```
