Consider the following code:

from mypy_extensions import TypedDict

class MovieBase(TypedDict):
    name: str
    year: int


class Movie(MovieBase, MovieBase):
    based_on: str

Notice that the Movie class inherits MovieBase twice.

mypy gives the following errors:

n.py:8: error: Cannot overwrite TypedDict field "name" while merging
n.py:8: error: Cannot overwrite TypedDict field "year" while merging

while there's no runtime error.

Note that if MovieBase was not a TypedDict, there would be a runtime TypeError: duplicate base class MovieBase

I think the right thing here would be to report duplicate base class.