BUG: race condition in Index.is_unique
#### Code Sample, a copy-pastable example if possible

Input:

```python
from concurrent.futures import ThreadPoolExecutor
import pandas as pd

x = pd.date_range('2001', '2020')
with ThreadPoolExecutor(2) as p:
    assert all(p.map(lambda x: x.is_unique, [x]*2))
```

Output:

```python
Traceback (most recent call last):
  File "bug.py", line 7, in <module>
    assert all(p.map(lambda x: x.is_unique, [x]*2))
AssertionError
```

#### Problem description

When calling `Index.is_unique` from multiple threads simultaneously, the wrong answer is returned.

#### Expected Output

Shouldn't raise.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.14.14-200.fc26.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: C
LANG: C
LOCALE: None.None

pandas: 0.23.0
pytest: 3.5.1
pip: 10.0.1
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 4.2.1
sphinx: 1.7.4
patsy: 0.5.0
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.2
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: 0.1.5
pandas_gbq: None
pandas_datareader: None
</details>
