(There is a small possibility that I have broken this in my branch securing the native interface so double check reproducible.)

On the master side the following error is reported: -

[Host Controller] 09:17:39,718 ERROR [stderr] (pool-3-thread-1) java.lang.IllegalArgumentException: There is already a registered server named 'slave'
[Host Controller] 09:17:39,719 ERROR [stderr] (pool-3-thread-1) at org.jboss.as.host.controller.DomainModelControllerService.registerChannel(DomainModelControllerService.java:384)
[Host Controller] 09:17:39,719 ERROR [stderr] (pool-3-thread-1) at org.jboss.as.host.controller.mgmt.NewMasterDomainControllerOperationHandlerImpl$RegisterOperation.writeResponse(NewMasterDomainControllerOperationHandlerImpl.java:123)
[Host Controller] 09:17:39,720 ERROR [stderr] (pool-3-thread-1) at org.jboss.as.protocol.mgmt.ManagementChannel$RequestReceiver.handleRequest(ManagementChannel.java:132)
[Host Controller] 09:17:39,720 ERROR [stderr] (pool-3-thread-1) at org.jboss.as.protocol.mgmt.ManagementChannel$RequestReceiver.access$300(ManagementChannel.java:108)
[Host Controller] 09:17:39,721 ERROR [stderr] (pool-3-thread-1) at org.jboss.as.protocol.mgmt.ManagementChannel.doHandle(ManagementChannel.java:62)
[Host Controller] 09:17:39,721 ERROR [stderr] (pool-3-thread-1) at org.jboss.as.protocol.ProtocolChannel.handleMessage(ProtocolChannel.java:149)
[Host Controller] 09:17:39,721 ERROR [stderr] (pool-3-thread-1) at org.jboss.remoting3.remote.RemoteConnectionChannel$4.run(RemoteConnectionChannel.java:206)
[Host Controller] 09:17:39,722 ERROR [stderr] (pool-3-thread-1) at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
[Host Controller] 09:17:39,722 ERROR [stderr] (pool-3-thread-1) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
[Host Controller] 09:17:39,723 ERROR [stderr] (pool-3-thread-1) at java.lang.Thread.run(Thread.java:662)

On the slave side.

The following line is logged multiple times: -

[Host Controller] 09:17:39,761 WARN [org.jboss.as.domain.controller] (Controller Boot Thread) Error retrieving domain model from remote domain controller 127.0.0.1:9999: There is already a registered server named 'slave'

Followed by this exception.

[Host Controller] 09:17:39,865 ERROR [stderr] (Controller Boot Thread) Exception in thread "Controller Boot Thread" java.lang.IllegalStateException: java.lang.Exception: There is already a registered server named 'slave'
[Host Controller] 09:17:39,865 ERROR [stderr] (Controller Boot Thread) at org.jboss.as.host.controller.NewRemoteDomainConnectionService.connect(NewRemoteDomainConnectionService.java:203)
[Host Controller] 09:17:39,865 ERROR [stderr] (Controller Boot Thread) at org.jboss.as.host.controller.NewRemoteDomainConnectionService.register(NewRemoteDomainConnectionService.java:136)
[Host Controller] 09:17:39,865 ERROR [stderr] (Controller Boot Thread) at org.jboss.as.host.controller.DomainModelControllerService.boot(DomainModelControllerService.java:275)
[Host Controller] 09:17:39,865 ERROR [stderr] (Controller Boot Thread) at org.jboss.as.controller.AbstractControllerService$1.run(AbstractControllerService.java:83)
[Host Controller] 09:17:39,866 ERROR [stderr] (Controller Boot Thread) at java.lang.Thread.run(Thread.java:662)
[Host Controller] 09:17:39,866 ERROR [stderr] (Controller Boot Thread) Caused by: java.lang.Exception: There is already a registered server named 'slave'
[Host Controller] 09:17:39,866 ERROR [stderr] (Controller Boot Thread) at org.jboss.as.host.controller.NewRemoteDomainConnectionService.connect(NewRemoteDomainConnectionService.java:199)
[Host Controller] 09:17:39,866 ERROR [stderr] (Controller Boot Thread) ... 4 more