Prevent duplicate ALIAS records from being added (#3446)

interface/web/dns/dns_a_edit.php

interface/web/dns/dns_aaaa_edit.php

interface/web/dns/dns_alias_edit.php

interface/web/dns/dns_cname_edit.php

interface/web/dns/dns_dname_edit.php

interface/web/dns/lib/lang/ar_dns_a.lng

interface/web/dns/lib/lang/ar_dns_aaaa.lng

interface/web/dns/lib/lang/ar_dns_alias.lng

interface/web/dns/lib/lang/ar_dns_cname.lng

interface/web/dns/lib/lang/bg_dns_a.lng

interface/web/dns/lib/lang/bg_dns_aaaa.lng

interface/web/dns/lib/lang/bg_dns_alias.lng

interface/web/dns/lib/lang/bg_dns_cname.lng

interface/web/dns/lib/lang/br_dns_a.lng

interface/web/dns/lib/lang/br_dns_aaaa.lng

interface/web/dns/lib/lang/br_dns_alias.lng

interface/web/dns/lib/lang/br_dns_cname.lng

interface/web/dns/lib/lang/ca_dns_a.lng

interface/web/dns/lib/lang/ca_dns_aaaa.lng

interface/web/dns/lib/lang/ca_dns_alias.lng