DNS secondary zone, confusion if zone exists as primary

DNS zones showed technical errors when tested, it turns out both ns1 and ns2 are configured as master of all zones. In ISPConfig panel ns2 is master and ns1 should have secondary zones. This is after migration, in SOURCE system ns1 was primary and ns2 had only secondary zones.
I do not know how this state was arrived at (I did not do the migration), and it was a bit laborious to discover what is happening. Since no zone transfers are being made the servers got out of sync.
What really masked the actual problem was when creating a secondary zone on ns1 did not change the situation, ns1 still answered with old data. I finally found ISPConfig datalog showed ns1 was made slave and checked named.conf.local. It had now two entries for the zone in question, one as master and in the end of the file the zone was marked as slave.
I think this is not worthy of a bug report, root cause is some mess up in migration. But it is worth knowing that ISPConfig when marking a zone as secondary does not check if the same zone already exists in named.conf.local. And does not remove the obsolete entry.
I'm removing on ns1 the primary zone entries and then in ISPConfig Panel create seconary zones.