Ensure `InitialValidators` have unique NodeIDs

[samliok]

From the PR comment

Is InitialValidators guaranteed to have unique NodeIDs? Without that guarantee there would appear to be a risk of the map not matching the slice with downstream impact on quorum. And maybe add a test validating the desired behavior when duplicate NodeIDs are present in the config?

Originally posted by @maru-ava in #4870 (comment)