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

```python
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()``

<details>


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]

</details>
