Duplicate headers

[JokerDevops]

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• I'm @tiangolo or he asked me directly to create an issue here.

Issue Content

Hi,
I like fastapi very much, but I encountered a problem during my study.

My code :

from typing import List, Union

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(x_token: Union[List[str], None] = Header(default=None)):
    return {"X-Token values": x_token}

I got a 422 return code not 200

According to the document description, I think I will get the following content:

{
    "X-Token values": [
        "bar",
        "foo"
    ]
}

[odiseo0]

Probably one of your other routes it's messing with it. Your example works for me

[JokerDevops]

Thank you for your reminder. I really didn't notice my other routes. It conflicts with other routes. I solved it. Now it is OK