Index.get_indexer() throws when None and at least one of np.nan, pd.NaT are present in input

[realead]

Code Sample

arr=pd.unique(np.array([pd.NaT, None], dtype=np.object))
index=pd.Index(arr, dtype=np.object).get_indexer([])

Problem description

throws "Reindexing only valid with uniquely valued Index objects".

This is also the case for [np.nan, None]

Expected Output

It should not crash. One would also expect, that the array returned by pd.unique() is really considered to be consisting of unique elements.

That is not the only problem in , but one blocker for #22305.

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-53-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 3.2.1
pip: 10.0.1
setuptools: 36.5.0.post20170921
Cython: 0.28.3
numpy: 1.13.1
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 0.9.8
lxml: 3.8.0
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: 0.1.3
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[paste the output of pd.show_versions() here below this line]

[realead]

The inconsistency is between which NA values are mangled in unique:

            # `val is None` below is exception to prevent mangling of None and

            # other NA values; note however that other NA values (ex: pd.NaT
            # and np.nan) will still get mangled, so many not be a permanent
            # solution; see GH 20866
            if not checknull(val) or val is None:
                k = kh_get_pymap(self.table, <PyObject*>val)
                if k == self.table.n_buckets:
                    kh_put_pymap(self.table, <PyObject*>val, &ret)
                    uniques.append(val)
            elif not seen_na:
                seen_na = 1
                uniques.append(nan)

and otherwise in the class, in particular in map_locations:

if val != val or val is None:
          val = na_sentinel