New Rule: no-duplicate-defined-urls

### Rule details

Warn when the same URL is defined with different identifiers.

### What type of rule is this?

Warns about a potential problem

### Example code

```markdown
[mercury]: https://example.com/mercury/
[venus]: https://example.com/mercury/
```

### Prior Art

[remark-lint-no-duplicate-defined-urls](https://github.com/remarkjs/remark-lint/tree/main/packages/remark-lint-no-duplicate-defined-urls)

### Participation

- [x] I am willing to submit a pull request to implement this rule.

### AI acknowledgment

- [x] I did not use AI to generate this issue report.
- [ ] (If the above is not checked) I have reviewed the AI-generated content before submitting.

### Additional comments

_No response_