Looks like in some rule another contact is defined.
In ~/var/log/notify.log it should list what rules a raw notification triggers and which rule (doesn’t) match. Then check the matched rules if contact conditions are set.
Alternatively, from the other way around, maybe the mail address of second contact can be found by doing a grep for all files in ~/etc/check_mk/conf.d/wato/, like @andreas-doehler suggested here to find an old tag in an unknown rule: Find unknown tags / no longer existing tags in rules - #2 by andreas-doehler
Regarding last notification, I assume there was no last notification, so it takes 0 as Unix timestamp. Time zero was on Thursday, January 1, 1970 00:00:00 UTC, but in your timezone probably +1.