Msgpack round trip changes duplicated() behavior for NaN's
#### Code Sample, a copy-pastable example if possible

```python
o = pd.Series(['test', np.nan, np.nan])
print 'Original has dupes: ', o.duplicated().any()

# Save and reload via msgpack
o.to_msgpack('/tmp/test.msgpack')
r = pd.read_msgpack('/tmp/test.msgpack')

print 'Reloaded has dupes: ', r.duplicated().any()

pd.testing.assert_series_equal(o, r)
print o.duplicated()
print r.duplicated()
```
#### Problem description

Current output:
```
Original has dupes:  True
Reloaded has dupes:  False
0    False
1    False
2     True
dtype: bool
0    False
1    False
2    False
dtype: bool
```

The series correctly has duplicates before sending to messagepack. Afterwards, the duplicated NaNs are not considered duplicates for some reason.

#### Output of ``pd.show_versions()``

<details>


INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-6-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.22.0
pytest: None
pip: None
setuptools: None
Cython: None
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 2.0.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: 2.5
feather: None
matplotlib: 1.5.2
openpyxl: None
xlrd: 0.9.3
xlwt: None
xlsxwriter: None
lxml: 3.4.4
bs4: None
html5lib: 1.0b8
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.0
</details>
