bind9 multiple RRs of singleon type

On an issue I have been working on a zone did not load on the bind9 server. When debugging I ran across the message "multiple RRs of singleon type". To find this error message I had to run named as "named -g -u bind -d 9". According to an article I found on this matter it is related to duplicate CNAMEs or SOA records on a zone.

I started out by looking into the zone file (sort zone.db) and quickly found two almost identical SOA records.

I deleted one line and after a quick reload of named everything started working.