BUG: rolling on centered datetimelike windows disrespect duplicate indices
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [X] I have confirmed this bug exists on the master branch of pandas.


### Reproducible Example

```python
import pandas as pd
index = pd.DatetimeIndex([
    '2020-01-01', '2020-01-01',
    '2020-01-02', '2020-01-02',
    '2020-01-03', '2020-01-03',
    '2020-01-04', '2020-01-04',
])
s = pd.Series([1]*8, index=index, dtype=float)

s.rolling('2D', center=True).sum()

>> Out[6]: 
2020-01-01    4.0
2020-01-01    4.0
2020-01-02    3.0
2020-01-02    4.0
2020-01-03    3.0
2020-01-03    4.0
2020-01-04    2.0
2020-01-04    2.0
dtype: float64
```


### Issue Description

the right window bound is not respected for **centered** datetime windows. 

This differs somewhat from issue #20712 and does not change the behavior discussed there..

i prepared a PR for this.  
 

### Expected Behavior

```python
s.rolling('2D', center=True).sum()
Out[4]: 
2020-01-01    4.0
2020-01-01    4.0
2020-01-02    4.0
2020-01-02    4.0
2020-01-03    4.0
2020-01-03    4.0
2020-01-04    2.0
2020-01-04    2.0
dtype: float64
```

### Installed Versions

<details>

INSTALLED VERSIONS
------------------
commit           : d30aeeba0c79fb8e4b651a8f528e87c3de8cb898
python           : 3.8.12.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-88-generic
Version          : #99-Ubuntu SMP Thu Sep 23 17:29:00 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8
pandas           : 1.4.0.dev0+857.gd30aeeba0c
numpy            : 1.21.2
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.2.4
setuptools       : 58.2.0
Cython           : 0.29.24
pytest           : 6.2.5
hypothesis       : 6.23.1
sphinx           : 4.2.0
blosc            : None
feather          : None
xlsxwriter       : 3.0.1
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.28.0
pandas_datareader: None
bs4              : 4.10.0
bottleneck       : 1.3.2
fsspec           : 2021.10.0
fastparquet      : 0.7.1
gcsfs            : 2021.10.0
matplotlib       : 3.4.3
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.9
pandas_gbq       : None
pyarrow          : 5.0.0
pyxlsb           : None
s3fs             : 2021.10.0
scipy            : 1.7.1
sqlalchemy       : 1.4.25
tables           : 3.6.1
tabulate         : 0.8.9
xarray           : 0.18.2
xlrd             : 2.0.1
xlwt             : 1.3.0
numba            : 0.53.1

</details>