BUG: Left join on index with non-unique values removes name of index #55815
Description
Pandas version checks
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Reproducible Example
import pandas as pd a = pd.DataFrame({"ID": [1, 2, 2], "a": [3, 4, 5]}) b = pd.DataFrame({"ID2": [1, 2], "b": [7, 8]}) a = a.set_index("ID") b = b.set_index("ID2") a = a.join(b) print(a.index.name)Issue Description
Left joining on an index with non-unique values in dataframe "a" resets the name of the index to None.
Expected Behavior
The column name should stay as "ID" in this case i.e. the name of the join column for dataframe "a".
Installed Versions
commit : e86ed37
python : 3.10.12.final.0
python-bits : 64
OS : Linux
OS-release : 5.10.60.1-microsoft-standard-WSL2
Version : #1 SMP Wed Aug 25 23:20:18 UTC 2021
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : C.UTF-8
LOCALE : en_US.UTF-8
pandas : 2.1.1
numpy : 1.24.3
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 59.6.0
pip : 22.0.2
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : None
pandas_datareader : None
bs4 : None
bottleneck : None
dataframe-api-compat: None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None