BUG: dataframe.fillna not working for dataframe with duplicate column names
### 

- [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
df1 = pd.DataFrame(pd.NA, index=[0, 1], columns=['A', 'A', 'B'])
df1.fillna({'A': 0})

df2 = pd.DataFrame(pd.NA, index=[0, 1], columns=['A', 'A'])
df2.fillna({'A': 0})
```


### Issue Description

For dataframes with duplicate column names `.fillna` fails to fill missing values, if a `dict` is passed as the `value` parameter

- **Example 1**  (`df1`).  When multiple, but not all, columns share a name, the `SettingWithCopyWarning` is thrown.
- **Example 2**  (`df2`).  When all column names are identical, no error is thrown, nor is missing values replaced.

### Expected Behavior

Expecting missing values in columns of the same name to be filled according to the parsed `dict`.

### Installed Versions

<details>

INSTALLED VERSIONS
------------------
commit           : 5f648bf1706dd75a9ca0d29f26eadfbb595fe52b
python           : 3.8.8.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : Danish_Denmark.1252
pandas           : 1.3.2
numpy            : 1.21.2
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 20.2.3
setuptools       : 49.2.1
Cython           : None
pytest           : 6.2.4
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 2021.08.1
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : 3.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.7.1
sqlalchemy       : 1.3.23
tables           : None
tabulate         : 0.8.9
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
