allow_redefinition ignored for functions

Bug Report

Mypy complains with error: Name "step_impl" already defined on line 13 error for functions, although allow_redefinition is set to True.

To Reproduce

1. Add mypy.ini configuration (listed below)
2. Add behave tests; for example:

@given("I am on the main page")
def step_impl(context):
    pass


@when("I type login and password")
def step_impl(context):
    pass


@then("I am logged in")
def step_impl(context):
    pass

3. Run mypy

Expected Behavior

Mypy checks pass because allow_redefinition is set to True.

Actual Behavior

Mypy fails with

 error: Name "step_impl" already defined on line 13

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
python_version = 3.6
allow_redefinition = True
ignore_missing_imports = True
check_untyped_defs = False

• Python version used: 3.6
• Operating system and version: Ubuntu 20.04.2

Start by reproducing the reported error with the shown mypy.ini configuration and behave-style repeated function definitions. Then trace mypy's handling of function redefinitions and add a regression test; done means allow_redefinition suppresses this diagnostic while unrelated duplicate-definition errors remain reported.