This rule is triggered if there are multiple headings in the document that have
the same text:

```markdown
# Some text

## Some text
```

To fix this, ensure that the content of each heading is different:

```markdown
# Some text

## Some more text
```

If the parameter `siblings_only` is set to `true`, duplication is allowed for
headings with different parents (as is common in changelogs):

```markdown
# Change log

## 1.0.0

### Features

## 2.0.0

### Features
```

Rationale: Some Markdown parsers generate anchors for headings based on the
heading name; headings with the same content can cause problems with that.