Function validate_public_keys 

async fn validate_public_keys(
    chain: &RegistrationChain,
    is_persistent: bool,
    report: &ProblemReport,
    context: &mut RbacBlockIndexingContext,
) -> Result<HashSet<VerifyingKey>>

Checks that a new registration doesn’t contain a signing key that was used by any other chain. Returns a list of public keys in the registration.