…avmic)

This PR was squashed before being merged into the 7.2 branch.

Discussion
----------

[Yaml] Throw on duplicate key even when value is NULL

| Q             | A
| ------------- | ---
| Branch?       | 5.4 (since v3.0)
| Bug fix?      | yes
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Issues        | Fix #57262
| License       | MIT

Duplicate keys are not valid by definition in YAML. The current implementation contains a bug that allows a key to be defined multiple times when the value is not set.

```yaml
services:
    Foo:
    Bar:
    Foo:
```

Extends symfony/yaml@8094454
to throw when a key is set twice in YAML without a value.

It may be technically a breaking change (as it suddenly makes some yaml-like files invalid), even though I'd classify it as a bugfix (as by definition, such files were not valid).

If we classify it as a bug, we should probably backport the fix to the oldest maintained version.

Commits
-------

f9df19b [Yaml] Throw on duplicate key even when value is NULL