Bug: multiple parameters with duplicate query works inconsistently

[cstarkers]

Description

When using Parameter(query=... to rename the parameter like this, it is possible to have the same query parameter map to multiple different Python parameters by setting query to an identical value.

Sometimes this works, and the provided HTTP query parameter gets copied into multiple Python parameters of different types. Sometimes it does not work, and a validation error is returned. Behaviour seems to depend on what types it is being coerced into.

I would expect that using the same query value to assign a HTTP query param to multiple Python params is either always allowed, or never allowed (and if it is not allowed, the current error message could be improved).

URL to code causing the issue

No response

MCVE

from litestar import Litestar, get
from litestar.params import Parameter
from typing import Annotated
from datetime import datetime

@get("/broken")
async def broken(
    some_date: Annotated[datetime, Parameter(query="someDate", required=True)],
    some_list: Annotated[
        list[str],
        Parameter(query="someDate", required=True),
    ],
) -> str:
    return f"{some_date=}, {some_list=}"

@get("/works")
async def works(
    some_date: Annotated[datetime, Parameter(query="someDate", required=True)],
    some_string: Annotated[
        str,
        Parameter(query="someDate", required=True),
    ],
) -> str:

    return f"{some_date=}, {some_string=}"

app = Litestar([broken, works])

Steps to reproduce

1. Run the MCVE
2. curl 'localhost:8000/works?someDate=2025-01-01T00:00:00Z' - this works.
3. curl 'localhost:8000/broken?someDate=2025-01-01T00:00:00Z' - this does not work, and returns a validation error.

Screenshots

No response

Logs

# curl 'localhost:8000/broken?someDate=2025-01-01T00:00:00Z'
{"status_code":400,"detail":"Validation failed for GET /broken?someDate=2025-01-01T00:00:00Z","extra":[{"message":"Expected `datetime`, got `array`","key":"some_date","source":"query"}]}%   

# curl 'localhost:8000/works?someDate=2025-01-01T00:00:00Z' 
some_date=datetime.datetime(2025, 1, 1, 0, 0, tzinfo=datetime.timezone.utc), some_string='2025-01-01T00:00:00Z'%

Litestar Version

2.16.0

Platform

• Linux
• Mac
• Windows
• Other (Please specify in the description above)

[cstarkers]

I will have some time in about a week and I am happy to look into this then. I am guessing the best answer is make it work in all cases - the alternative would be to make it always an error, which introduces a backwards incompatibility