pivot may still introduce duplicate column names (in a different way!)

[MarcoGorelli]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

the duplicates never stop

In [6]: df = pl.DataFrame({'a': [1,3,2], 'c': ['a', 'a', 'a'], 'd':[7,8,9]})

In [7]: df.pivot(index='a', columns='c', values='d')
Out[7]:
shape: (3, 2)
┌─────┬─────┐
│ a   ┆ a   │
│ --- ┆ --- │
│ i64 ┆ i64 │
╞═════╪═════╡
│ 1   ┆ 7   │
│ 3   ┆ 8   │
│ 2   ┆ 9   │
└─────┴─────┘

Log output

No response

Issue description

if index is a column with the same name as one of the values in the columns column

Expected behavior

raise

Installed versions

In [4]: pl.show_versions()
--------Version info---------
Polars:               0.20.7
Index type:           UInt32
Platform:             Linux-5.15.133.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
Python:               3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          3.0.0
connectorx:           <not installed>
deltalake:            <not installed>
fsspec:               2023.12.2
gevent:               <not installed>
hvplot:               0.9.1
matplotlib:           3.8.2
numpy:                1.26.3
openpyxl:             <not installed>
pandas:               2.2.0
pyarrow:              15.0.0
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           2.0.25
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[ritchie46]

Any idea of a naming strategy here?

[MarcoGorelli]

tidyr would just raise, and pandas doesn't consider duplicate column names to be a bug (!) (though pandas has an index anyway, so in this case it would just name the index 'a')

I'd suggest just raising