Is your feature request related to a problem

Yes.

Given some URL encoded data like this...

choices=parrot&choices=spider

...only the last key=value wins.

This does not work like I expected:

choices: list = Form(...)

You can only validate against the last value.

The solution you would like

Perhaps FastAPI should collect repeated keys in the 2-tuple list that request.form() gives and assign those values as a list to the same key before validation happens.