[`ruff`] Detect duplicate entries in `__all__` (`RUF068`) (#22114)
Hello,
This MR adds a new rule and its fix, `RUF069`,
`DuplicateEntryInDunderAll`. I'm using `RUF069` because we already have
[RUF068](https://github.com/astral-sh/ruff/pull/20585) and
[RUF069](https://github.com/astral-sh/ruff/pull/21079#issuecomment-3493839453)
in the works.
The rule job is to prevent users from accidentally adding duplicate
entries to `__all__`, which, for example, can result from copy-paste
mistakes.
It deals with the following syntaxes:
```python
__all__: list[str] = ["a", "a"]
__all__: typing.Any = ("a", "a")
__all__.extend(["a", "a"])
__all__ += ["a", "a"]
```
But it does not keep track of `__all__` contents, meaning the following
code snippet is a false negative:
```python
class A: ...
__all__ = ["A"]
__all__.extend(["A"])
```
## Violation Example
```console
RUF069 `__all__` contains duplicate entries
--> RUF069.py:2:17
|
1 | __all__ = ["A", "A", "B"]
| ^^^
help: Remove duplicate entries from `__all__`
1 | __all__ = ["A", "B"]
- __all__ = ["A", "A", "B"]
```
## Ecosystem Report
The `ruff-ecosystem` results contain seven violations in four projects,
all of them seem like true positives, with one instance appearing to be
an actual bug.
This [code
snippet](https://github.com/python/typeshed/blob/90d855985be5aae9bc76e77b0f3d4b6738c38347/stubs/reportlab/reportlab/lib/rltempfile.pyi#L4)
from `reportlab` contains the same entry twice instead of exporting both
functions.
```python
def get_rl_tempdir(*subdirs: str) -> str: ...
def get_rl_tempfile(fn: str | None = None) -> str: ...
__all__ = ("get_rl_tempdir", "get_rl_tempdir")
```
Closes [#21945](https://github.com/astral-sh/ruff/issues/21945)
---------
Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com>
