A CNAME record cannot exist with the same owner name as another record
except for its RRSIG and the DNSSEC records which prove non-existence of other records (NSEC).
RFC 1034, Section 3.6.2: "If a CNAME RR is present at a node, no
other data should be present; this ensures that the data for a
canonical name and its aliases cannot be different. This rule also
insures that a cached CNAME can be used without checking with an
authoritative server for other RR types."