adshmh: Check for duplication in presubmits' extra_refs in checkconfig

JobBuild
15372
11-22 23:14
pull-test-infra-bazel 1199428079643004929 1198986356328501248 1198016839079170050
pull-test-infra-bazel-canary 1198986356332695553 1198016839079170048
pull-test-infra-verify-file-perms 1199428079643004930 1198986356332695552 1198016839079170049
pull-test-infra-yamllint 1199428079643004928 1198986356332695554 1198016839079170051