Add a test to stop adding values that already exist

[eskes1]

To test
Add test.domain.com as A record
Add test.domain.com as CNAME
That should give an error.

[morremeyer]

While this logically does not make any sense as you noted, I am not sure if this is actually forbidden in any standard. If that is the case, this should throw an error. If however this is not forbidden, this should at maximum warn the user about this action. Regards Morre

[Z9n2JktHlZDmlhSvqc9X2MmL3BwQG7tk]

I am not sure if this is actually forbidden in any standard.

In case of CNAME it is forbidden: CNAME records can not have any other types.

https://www.ietf.org/rfc/rfc1912.txt

2.4 CNAME records

A CNAME record is not allowed to coexist with any other data. In
other words, if suzy.podunk.xx is an alias for sue.podunk.xx, you
can't also have an MX record for suzy.podunk.edu, or an A record, or
even a TXT record. Especially do not try to combine CNAMEs and NS
records like this!:

       podunk.xx.      IN      NS      ns1
                       IN      NS      ns2
                       IN      CNAME   mary
       mary            IN      A       1.2.3.4

This is often attempted by inexperienced administrators as an obvious
way to allow your domain name to also be a host. However, DNS
servers like BIND will see the CNAME and refuse to add any other
resources for that name. Since no other records are allowed to
coexist with a CNAME, the NS entries are ignored. Therefore all the
hosts in the podunk.xx domain are ignored as well!