Prevent duplicate records in rrsets

[jsoref]

Apparently I made a mistake in August 2023. Thankfully dns-ui keeps a log of my actions:

from bottom to top: two events with the same timestamp and content; a later one that appears to show that both of the previous events resulted in the same content being added twice to the RRSet, and then one of them was removed in the later update.

ideally, dns-ui would have prevented the duplication, perhaps even quietly