check-for-live-server attribute should be set on true by default, otherwise failback will not work by default and lots of this messages will spam logs on both servers :
15:01:32,379 WARN [org.apache.activemq.artemis.core.client] (activemq-discovery-group-thread-dg-group1) AMQ212034: There are more than one servers on the network broadcasting the same node id. You will see this message exactly once (per node) if a node is restarted, in which case it can be safely ignored. But if it is logged continuously it means you really do have more than one node on the same network active concurrently with the same node id. This could occur if you have a backup node active at the same time as its live node. nodeID=4b420277-91ea-11e5-ba47-5d09590e6c44

Note: changing this value is not possible on ER1 because of https://issues.jboss.org/browse/JBEAP-1980. But with DR12 it works.