enigmatic "name already defined" error

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?

Bug.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

There are four pieces of code:

def func(x: int) -> int:
    return x + 1

def func(x):
    return x

def func(x):
    return x

def func(x: int) -> int:
    return x + 1

from test import func

def func(x: int) -> int:
    return x + 1

def func(x: int) -> int:
    return x + 1

from test import func

Whenever test is used as a module, test.py contains

def func(x):
    return x

• What is the actual behavior/output?

The first three examples emits "Name 'func' already defined" error, but not the last

• What is the behavior/output you expect?

All four should emit this error.

• What are the versions of mypy and Python you are using?

mypy 0.720+dev.dc6da54f50d5443fe46308046719043eba03c84d (master at the time of bug report)
Python 3.7.3

• What are the mypy flags you are using? (For example --strict-optional)

None.

まず、報告された mypy のバージョンで 4 つのインライン再現を実行し、インポートされる関数には test.py を使用します。各インポート順序と再定義順序について診断結果を比較します。4 つすべてのケースで duplicate-definition エラーが出力されるというレポートの期待どおりの動作になれば完了です。