In the case where the DNS options of RDNSS and DNSSL can be obtained
   from multiple sources, such as RA and DHCP, the IPv6 host SHOULD keep
   some DNS options from all sources.  Unless explicitly specified for
   the discovery mechanism, the exact number of addresses and domain
   names to keep is a matter of local policy and implementation choice.
   However, the ability to store at least three RDNSS addresses (or
   DNSSL domain names) from at least two different sources is
   RECOMMENDED.  The DNS options from Router Advertisements and DHCP
   SHOULD be stored into the DNS Repository and Resolver Repository so
   that information from DHCP appears there first and therefore takes
   precedence.  Thus, the DNS information from DHCP takes precedence
   over that from RA for DNS queries.  On the other hand, for DNS
   options announced by RA, if some RAs use the Secure Neighbor
   Discovery (SEND) protocol [RFC3971] for RA security, they MUST be
   preferred over those that do not use SEND.  Refer to Section 7 for
   the detailed discussion on SEND for RA DNS options.