BUG: Series.loc slicing raises KeyError unexpectedly

[DominikHallab]

Pandas version checks

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Reproducible Example

import pandas as pd

print(pd.Series([3, 6, 7, 6], index=[3, 6, 7, 9]).loc[5:], "\n")  # works
print(pd.Series([3, 6, 7, 6], index=[3, 6, 7, 6]).loc[5:], "\n")  # raises error

Issue Description

When an index value is repeated, loc slicing raises a KeyError.
The documentation does not mention this as a condition for raising a KeyError.

Expected Behavior

I think this should return d.Series([6, 7, 6], index=[6, 7, 6]) and not raise an error.

Installed Versions

INSTALLED VERSIONS
------------------
commit                : 9c8bc3e55188c8aff37207a74f1dd144980b8874
python                : 3.10.11
python-bits           : 64
OS                    : Windows
OS-release            : 10
Version               : 10.0.26100
machine               : AMD64
processor             : Intel64 Family 6 Model 186 Stepping 3, GenuineIntel
byteorder             : little
LC_ALL                : None
LANG                  : en_US.UTF-8
LOCALE                : English_New Zealand.1252

pandas                : 2.3.3
numpy                 : 2.2.6
pytz                  : 2025.2
dateutil              : 2.9.0.post0
pip                   : 23.0.1
Cython                : None
sphinx                : None
IPython               : None
adbc-driver-postgresql: None
adbc-driver-sqlite    : None
bs4                   : None
blosc                 : None
bottleneck            : None
dataframe-api-compat  : None
fastparquet           : None
fsspec                : None
html5lib              : None
hypothesis            : None
gcsfs                 : None
jinja2                : None
lxml.etree            : None
matplotlib            : None
numba                 : None
numexpr               : None
odfpy                 : None
openpyxl              : None
pandas_gbq            : None
psycopg2              : None
pymysql               : None
pyarrow               : None
pyreadstat            : None
pytest                : None
python-calamine       : None
pyxlsb                : None
s3fs                  : None
scipy                 : None
sqlalchemy            : None
tables                : None
tabulate              : None
xarray                : None
xlrd                  : None
xlsxwriter            : None
zstandard             : None
tzdata                : 2025.2
qtpy                  : None
pyqt5                 : None
None

[jakechirsch]

I would like to fix this one

[Lonercode]

It appears the index has to be sorted for it to work. That's why the second print statement fails. Found something in the docs on that you could check out: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#slicing-with-labels

[dogedede]

When the index contains duplicate values, pandas .loc[] slice operation cannot properly determine the range. Consider using boolean filtering:

s = pd.Series([3, 6, 7, 6], index=[3, 6, 7, 6])
result = s[s.index >= 5]
print(result)

Or directly use .iloc[] to select data by position:
result = s.iloc[2:]