Duplicate input keys (includes or inline) should throw error in the UI

The following discussion from !197684 (closed) should be addressed:

• @furkanayhan started a discussion: (+7 comments)

  What would happen in this scenario?

  # .gitlab-ci.yml
  
  spec:
    inputs:
      abc:
        type: number
        default: 3
  
  ---
  
  include:
    - local: template.yml
      inputs:
        environment: "production"
        app_version: "2.0.0"
        replicas: 5

  template.yml

  spec:
    include:
      - remote: asdasd.yml
    inputs:
      environment:
        default: "staging"
        options: ["development", "staging", "production"]
      app_version:
        default: "1.0.0"
      replicas:
        type: number
        default: 3
  
  ---
  
  test:
    stage: test
    script:
      - echo "Running tests"