(🐞) Duplicate error when `show-error-context` is enabled

```py
def foo(a: object = b) -> None:
    pass
```
```
main.py:1: error: Name "b" is not defined  [name-defined]
main.py: note: In function "foo":
main.py:1: error: Name "b" is not defined  [name-defined]
```
[playground](https://mypy-play.net/?mypy=latest&python=3.11&flags=show-error-context&gist=bc7e6b9eed96dbc4fa6577738f8fd401)