Fix numpy warning
Various failures like

```
=================================== FAILURES ===================================

_______________ TestIndex.test_isin_nan_common_object[nan0-nan2] _______________

[gw0] linux -- Python 3.8.3 /home/travis/miniconda3/envs/pandas-dev/bin/python

self = <pandas.tests.indexes.test_base.TestIndex object at 0x7ff81c8b2e80>

nulls_fixture = nan, nulls_fixture2 = nan

    def test_isin_nan_common_object(self, nulls_fixture, nulls_fixture2):

        # Test cartesian product of null fixtures and ensure that we don't

        # mangle the various types (save a corner case with PyPy)

    

        # all nans are the same

        if (

            isinstance(nulls_fixture, float)

            and isinstance(nulls_fixture2, float)

            and math.isnan(nulls_fixture)

            and math.isnan(nulls_fixture2)

        ):

>           tm.assert_numpy_array_equal(

                Index(["a", nulls_fixture]).isin([nulls_fixture2]),

                np.array([False, True]),

            )
```

which I can't look into at the moment. I'll just remove the duplicate key.