So reading the links from the Symfony Yaml component where there are tests for this behaviour it links to http://yaml.org/spec/1.1/#id932806

It is an error for two equal keys to appear in the same mapping node. In such a case the YAML processor may continue, ignoring the second key: value pair and issuing an appropriate warning.

So what I think we are missing is and issuing an appropriate warning

This is the PR that introduced this behavior: #10902 and in other issues we've been discussing about this behavior too: #14512.

@xabbuh is the main Yaml component maintainer, so let's see what he thinks about introducing changes related to this. Thanks!

Would it be possible to use a silenced warning or a flag to throw/warn on a duplicate key?

@sstok I was just coding up a PR to introduce a new parse flag PARSE_EXCEPTION_ON_DUPLICATE

I guess that the current behaviour was implemented based on the assumption that duplicate keys are allowed (see the quoted part of the spec at #19526 (comment)). However, as you can this part is from the YAML 1.1 spec, but we do follow the YAML 1.2 which does forbid duplicate keys (see the issue description).

With this in mind I suggest to deprecate the current behaviour in Symfony 3.2 and throw a ParsException starting with Symfony 4.0.

@symfony/deciders What are your thoughts on this?

@xabbuh I agree: deprecate in 3.2; exception in 4.0.

I agree too. Duplicate keys can be a pain to debug anyway.

Does this effect the ability to include other config files? Like how the config_prod.yml file loads the config.yml file? Or is that a different concept that does that?

@patrick-mcdougle That's a different concept which is specific to how the DependencyInjection and Routing component handle the merging of configuration files that have been parsed by the parser from the Yaml component.