From support ticket:

The case is following(with imaginary ip address):

1) We have node1 with ip address 10.10.10.10 which is primary interface on that node in OpenNMS
2) We have node2 with same ip address 10.10.10.10 which is not primary interface on that node in OpenNMS
3) Then SNMP trap arrives from node 1 and this trap is mapped to node 2 in OpenNMS with this 10.10.10.10 address which is not even primary interface.

So they way this should go in my opinion is that this should map to the node which has primary interface on this address. At least in our case. I also queried this interface address from the OpenNMS DB and select is returning first this wrong node ip interface, so is alarm just mapped based on which node address returns first?