Summary

Tighten the boilerplate check so files with duplicate Kubernetes license headers are rejected instead of silently passing.

Changes

• ignore .cache while walking the repository for boilerplate checks
• reject files that contain the Kubernetes copyright header more than once
• add the missing boilerplate header to hack/lint-links.sh
• remove duplicate boilerplate blocks from hack/check-capi-version.sh and test/helpers/mod.go
• add a focused unit test for duplicate-header detection
• keep the shell test fixture failing for the intended reason so the ensure path stays covered

Testing

• python -m unittest boilerplate_test in hack/boilerplate
• make verify-boilerplate