Bug: openjd check does not handle duplicate keys

[AWS-Samuel]

Describe the bug

Given a job template, which has duplicate keys. openjd check will consider the job template as valid, even though it's not valid yaml.

Expected Behaviour

The template will not be considered valid, due to having duplicate keys

Current Behaviour

The template will be considered valid, even with duplicate keys.

Reproduction Steps

Given the template:

specificationVersion: 'jobtemplate-2023-09'
specificationVersion: 'jobtemplate-2023-09'
name: Foo
name: Foo
steps:
- name: Bar
  script:
    actions:
      onRun:
        command: Baz
        command: Baz
      onRun:
        command: Baz
        command: Baz
    actions:
      onRun:
        command: Baz
        command: Baz
      onRun:
        command: Baz
        command: Baz

> openjd check template.yaml
Template at 'template.yaml' passes validation checks.