[Yaml] Duplicate keys not reported when key is defined with NULL value #57262
Description
Symfony version(s) affected
7+, 6+, 5+
Description
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.
How to reproduce
services:
Foo:
Bar:
Foo:is invalid YAML but the YAML parser is fine with this.
Possible Solution
No response
Additional Context
No response