Explode on identical index causes duplication
#### Code Sample, a copy-pastable example if possible

```python
>>> df = pd.DataFrame({'col': [['a', 'b'], ['c', 'd']]}, index=[0,0])
>>> df.explode('col')
  col
0   a
0   b
0   c
0   d
0   a
0   b
0   c
0   d
```
#### Problem description
I'm not sure if this is expected behaviour. But it took me slightly off guard when I had a datetime index with occasionally identical timestamps. This is the output I expected:
```python
  col
0   a
0   b
0   c
0   d
```
Thanks for your work on pandas! Apologies if this is actually normal behaviour.

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.7.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.4.0-139-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US
LOCALE           : en_US.UTF-8

pandas           : 0.25.0
numpy            : 1.14.3
pytz             : 2018.9
dateutil         : 2.8.0
pip              : 19.0.2
setuptools       : 40.8.0
Cython           : None
pytest           : 4.5.0
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 7.3.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.0.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.2.1
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

</details>
