[Yaml] Fixed YAML Parser does not ignore duplicate keys, violating YAML spec.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | **mayhaps**
| Deprecations? | no
| Tests pass?   | yes
| Needs merge to | 2.4
| Fixed tickets | —
| License       | MIT
| Doc PR        | —

The current [YAML 1.2] specification clearly states:

> JSON's RFC4627 requires that mappings keys merely “SHOULD” be unique, while YAML insists they “MUST” be.

The outdated [YAML 1.1] spec contained a crystal clear note on how the error of duplicate keys is to be handled by parsers, which is (sadly) no longer contained in the latest 1.2 spec (only leaving the requirement):

> 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.  This strategy preserves a consistent information model for one-pass and random access applications.

[YAML 1.2]: http://yaml.org/spec/1.2/spec.html#id2759572
[YAML 1.1]: http://yaml.org/spec/1.1/#id932806