I'd love this feature. In a few big applications with 25+ developers, mistakes like this happen every once in a while. I love to see them crash instead of silently continuing.

As proposed by other people, i would prefer deprecate this and throw an exception in symfony 4.0.

@alexpott OK to replace this flag by a deprecation?

The failure in fabbot.io is on a line not changed by this patch - I guess src/Symfony/Component/Yaml/Tests/Fixtures/YtsSpecificationExamples.yml does not comply with the standard.

👍

Please also update the upgrade files for 3.2 and 4.0.

Thank you@alexpott.

IIRC, the Yaml spec explicitly says that parsers should use the first key when finding duplicates (and we made a bugfix in the past to respect it instead of using the last one). So this deprecation is a violation of the spec

@stof from what i see, the spec says that it should be considered as an error:

If both notations are used as keys in the same mapping, only a YAML processor which recognizes integer formats would correctly flag the duplicate key as an error.

And:

Technically, YAML therefore complies with the JSON spec, choosing to treat duplicates as an error. In practice, since JSON is silent on the semantics of such duplicates, the only portable JSON files are those with unique keys, which are therefore valid YAML files.

Good day!
Was looking for exact same feature. It is said to be in the master, but I cannotfind it. Am I missing something?

@ivangretsky There is no flag controlling this. The parser just triggers a deprecation when detecting duplicated keys on 3.2 or higher and will throw an exception starting with Symfony 4.0.