Override class-level fixture parametrization with method-level parametrization

What's the problem this feature will solve?

Please see the below Python 3.8 code snippet. It involves:

• Fixture being indirectly parametrized
  • Class parametrization: idea is this parameterization ("b") applies to all tests
  • Method parameterization: idea is this parameterization ("a") applies to just a particular method

from typing import Any

import pytest

@pytest.fixture
def some_fixture(request) -> Any:
    assert "a" in request.param  # Only passes for test_something
    assert "b" in request.param  # Always passes

# I apply to all test methods
@pytest.mark.parametrize("some_fixture", [{"b": "b"}], indirect=True)
class TestMultiParameterization:

    # I apply to just this one test method
    @pytest.mark.parametrize("some_fixture", [{"a": "a"}], indirect=True)
    def test_something(self, some_fixture: Any) -> None:
        pass

Running the test_something method via pytest==6.2.5, it will spit out the following error:

E   ValueError: duplicate 'some_fixture'

Clearly, pytest rejects the duplicated parameterization.

Describe the solution you'd like

I would like if one could somehow "stitch together" parameterization from class-level parameterizations and method-level parameterizations.

The main benefit would be increased flexibility in parameterization of test fixtures. I am not sure how this could be accomplished in practice, though I do think it'd be useful functionality.

Alternative Solutions

I have not tried any alternate solutions.

Additional context

Start by reproducing the class- and method-level fixture parametrization example with pytest 6.2.5 and confirm the duplicate 'some_fixture' error. Investigate pytest's parametrization handling, then define behavior where method-level parametrization overrides or combines with the class-level value while preserving the stated fixture assertions.