Usage of .loc with output of drop_dupilcates() returned DatetimeIndex
Basically I want to get rid of duplicated DatetimeIndex items. I do not understand why this does not work using the output of "drop_duplicates()".  

I am using pandas 0.24.2

This shows that drop_duplicates() in the loc statement does not work although it does work with the boolean approach.

```python
import pandas as pd

df1 = pd.DataFrame(index=pd.date_range("2018-01-01", periods=3))
df2 = pd.DataFrame(index=pd.date_range("2018-01-03", periods=3))
df3 = df1.append(df2)
df3 = df3.loc[df3.index.drop_duplicates(), :]
print(df3)
# Empty DataFrame
# Columns: []
# Index: [2018-01-01 00:00:00, 2018-01-02 00:00:00, 2018-01-03 00:00:00, 2018-01-03 00:00:00, 2018-01-04 00:00:00, 2018-01-05 00:00:00]
print(df3.loc[~df3.index.duplicated(), :])
# Empty DataFrame
# Columns: []
# Index: [2018-01-01 00:00:00, 2018-01-02 00:00:00, 2018-01-03 00:00:00, 2018-01-04 00:00:00, 2018-01-05 00:00:00]

print(df3.index.drop_duplicates())
# DatetimeIndex(['2018-01-01', '2018-01-02', '2018-01-03', '2018-01-04',
#               '2018-01-05'],
#              dtype='datetime64[ns]', freq=None)
```
drop_duplicates() returns the correct indexes, but does not work as expected in .loc. 

So, first thing I thought is that I can not use an index to select rows and that I instead need to always use boolean lists/arrays/…. Then, I checked types and tried to use an index range, which works, surprisingly:

```python
print(type(df3.index.drop_duplicates()))
# <class 'pandas.core.indexes.datetimes.DatetimeIndex'>
```

```python
print(type(df3.index[2:4]))
# <class 'pandas.core.indexes.datetimes.DatetimeIndex'>
```


```python
print(df3.loc[df3.index[1:3],:])
# Empty DataFrame
# Columns: []
# Index: [2018-01-02 00:00:00, 2018-01-03 00:00:00, 2018-01-03 00:00:00]

```


So, I can use .loc to get certain indexes, but although the type is the same, I can not use the output of drop_duplicates(), which is not what I expected.

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.3.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.2
pytest: 4.4.1
pip: 19.1
setuptools: 41.0.1
Cython: None
numpy: 1.16.3
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2019.1
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
</details>
