Using package multipledispatch, mypy gives error name already defined

Bug Report

Mypy reports errors when using package multipledispatch for overloading functions.

To Reproduce

from multipledispatch import dispatch


@dispatch(list, str)
def concatenate(a: list, b: str):
    a.append(b)
    return a


@dispatch(str, str)
def concatenate(a: str, b: str):
    return a + b


@dispatch(str, int)
def concatenate(a: str, b: int):
    return a + str(b)


@dispatch(int, str)
def concatenate(a: int, b: str):
    return str(a) + b


print(concatenate(["a", "b"], "c"))
# ['a', 'b', 'c']
print(concatenate("Hello", "World"))
# HelloWorld
print(concatenate("a", 1))
# a1
print(concatenate(2, "b"))
# 2b

Expected Behavior

No errors

Actual Behavior

mypy multi.py

multi.py:10: error: Name "concatenate" already defined on line 4
multi.py:15: error: Name "concatenate" already defined on line 4
multi.py:20: error: Name "concatenate" already defined on line 4

(Write what happened.)

Your Environment

• Mypy version used: 0.910
• Python version used: 3.8.6
• Operating system and version: macOS 11.5.2
• Installed packages:
  flake8==3.9.2
  mccabe==0.6.1
  multipledispatch==0.6.0
  mypy==0.910
  mypy-extensions==0.4.3
  pycodestyle==2.7.0
  pyflakes==2.3.1
  six==1.16.0
  toml==0.10.2
  typing-extensions==3.10.0.2

Start by running the reported mypy multi.py example with the shown multipledispatch definitions and confirm the repeated-name diagnostics. Trace the name-checking path for these repeated function definitions and compare it with existing overload handling; done means the example type-checks without those errors while ordinary duplicate definitions remain diagnosed.