TST: DF should not error when 'columns' is a list that contains dupes

[ghost]

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

[ghost]

just the failing test.

[ghost]

converted to #2079