dnssec: keys with duplicate keytags problems

Reported by @KlepalJan_twitter: "It seems there is a bug with automatic zone signing. Upon knot reload or re-signing zone ZSK record (DNSKEY 256 3 5) disappears from zone. Upon next reload or re-sign record again appears then again disappears and so on. The problem seems to be caused by duplicate keytag in the kasp/zone_domain.cz.json file. Workaround is to delete old key record with duplicate keytag from JSON file. I found note in gitlab issue #2 (closed) "TODOs until 1.4.0: Do not allow keys with identical keytags." however I was using version 2.3.0 (upgrade to 2.3.3 did not fix the problem). It was coincidence (keys were generated by knot autosigning process)."