When an ImageSetConfiguration contains duplicate top-level keys (e.g., two mirror: blocks), oc-mirror v2 parses the file without any error. The YAML parser (sigs.k8s.io/yaml) silently discards earlier keys and only applies the last one.
This results in misleading success output while portions of the configuration are ignored. For example, a platform section can be dropped entirely if followed by another mirror: section defining operators.    

    1. Create an imageset.yaml with two mirror: blocks:

kind: ImageSetConfiguration
apiVersion: mirror.openshift.io/v2alpha1
mirror:
  platform:
    channels:
      - name: stable-4.18
        minVersion: 4.18.20
        maxVersion: 4.18.21
    graph: truemirror:
  operators:
    - catalog: registry.redhat.io/redhat/redhat-operator-index:v4.18
      packages:
        - name: metallb
  additionalImages:
    - name: registry.redhat.io/ubi8/ubi:latest

2. Run oc mirror.
3. The command completes successfully.
4. Only the operators and additionalImages are mirrored; the platform images are silently ignored.     

oc mirror runs successfully without warnings or errors.
Only the last mirror: block is honored.
Earlier sections (e.g., platform) are silently dropped.    

oc mirror should fail fast with a clear validation error.
Duplicate top-level keys in ImageSetConfiguration should be rejected.
Schema validation should enforce correctness of the configuration before mirroring begins.