TST: DF should not error when 'columns' is a list that contains dupes
test_getitem_list_duplicates() shows that the same works for numpy arrays.

> In [1]:  DataFrame(data,columns=["a","a"])
> 
> ...
> pandas/pandas/core/internals.pyc in _stack_dict(dct, ref_items, dtype)
>  1344     stacked = np.empty(shape, dtype=dtype)
> 1345     for i, item in enumerate(items):
> -> 1346         stacked[i] = _asarray_compat(dct[item])
>   1347 
>   1348     # stacked = np.vstack([_asarray_compat(dct[k]) for k in items])
> 
> IndexError: index out of bounds
