Indexing MultiIndex with duplicates return Series instead of raw value 
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

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

---

#### Code Sample, a copy-pastable example
```
single_index_no_duplicates = pd.Index([0, 1, 2])
single_index_with_duplicates = pd.Index([0, 1, 1])

multi_index_no_duplicates = pd.MultiIndex.from_tuples([(0, 0), (0, 1), (1, 0)])
multi_index_with_duplicates = pd.MultiIndex.from_tuples([(0, 0), (1, 0), (1, 0)])

>>> pd.Series(range(3), single_index_no_duplicates).loc[0]
0
>>> pd.Series(range(3), single_index_with_duplicates).loc[0]
0
>>> pd.Series(range(3), multi_index_no_duplicates).loc[(0,0)]
0
>>> pd.Series(range(3), multi_index_with_duplicates).loc[(0,0)]
0  0    0
dtype: int64

```

#### Problem description

When indexing a unique index value with `.loc` on a `MultiIndex` containing duplicated values,
the return type is a `Series `while it is a raw value for single `Index`. 

#### Expected Output

I think we should expect a raw value to keep consistent with `Index` behavior.

```
>>> pd.Series(range(3), multi_index_with_duplicates).loc[(0,0)]
0
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.8.0-55-generic
Version          : #62~20.04.1-Ubuntu SMP Wed Jun 2 08:55:04 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : fr_FR.UTF-8
LOCALE           : fr_FR.UTF-8

pandas           : 1.2.4
numpy            : 1.20.3
pytz             : 2019.3
dateutil         : 2.7.3
pip              : 20.0.2
setuptools       : 45.2.0
Cython           : None
pytest           : 6.2.4
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
