Description of problem:
Trying to extend a storage domain (tried in O-S-T) fails with (engine.log):
2018-06-06 07:03:06,526-04 ERROR [org.ovirt.engine.core.vdsbroker.vdsbroker.ConnectStorageServerVDSCommand] (default task-2) [2a808cc4] Command 'ConnectStorageServerVDSCommand(HostName = lago-basic-suite-master-
host-1, StorageServerConnectionManagementVDSParameters:{hostId='4d59833e-7b64-46ec-9d98-39104f5e04ec', storagePoolId='b85207b2-73d6-4b02-818d-d93871e80692', storageType='ISCSI', connectionList='[StorageServerCon
nections:{id='null', connection='192.168.200.4', iqn='iqn.2014-07.org.ovirt:storage', vfsType='null', mountOptions='null', nfsVersion='null', nfsRetrans='null', nfsTimeo='null', iface='null', netIfaceName='null'
}, StorageServerConnections:{id='null', connection='192.168.201.4', iqn='iqn.2014-07.org.ovirt:storage', vfsType='null', mountOptions='null', nfsVersion='null', nfsRetrans='null', nfsTimeo='null', iface='null', 
netIfaceName='null'}]', sendNetworkEventOnFailure='true'})' execution failed: Duplicate key 0
2018-06-06 07:03:06,526-04 DEBUG [org.ovirt.engine.core.vdsbroker.vdsbroker.ConnectStorageServerVDSCommand] (default task-2) [2a808cc4] Exception: java.lang.IllegalStateException: Duplicate key 0
        at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133) [rt.jar:1.8.0_171]
        at java.util.HashMap.merge(HashMap.java:1254) [rt.jar:1.8.0_171]
        at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320) [rt.jar:1.8.0_171]
        at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169) [rt.jar:1.8.0_171]
        at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948) [rt.jar:1.8.0_171]
        at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) [rt.jar:1.8.0_171]
        at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) [rt.jar:1.8.0_171]
        at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) [rt.jar:1.8.0_171]
        at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) [rt.jar:1.8.0_171]
        at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) [rt.jar:1.8.0_171]
        at org.ovirt.engine.core.vdsbroker.vdsbroker.ServerConnectionStatusReturn.convertToStatusList(ServerConnectionStatusReturn.java:44) [vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.vdsbroker.ConnectStorageServerVDSCommand.executeVdsBrokerCommand(ConnectStorageServerVDSCommand.java:49) [vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.vdsbroker.VdsBrokerCommand.executeVdsCommandWithNetworkEvent(VdsBrokerCommand.java:123) [vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.vdsbroker.ConnectStorageServerVDSCommand.executeVDSCommand(ConnectStorageServerVDSCommand.java:41) [vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.VDSCommandBase.executeCommand(VDSCommandBase.java:65) [vdsbroker.jar:]
        at org.ovirt.engine.core.dal.VdcCommandBase.execute(VdcCommandBase.java:31) [dal.jar:]



It seem to have succeeded on the host:
2018-06-06 07:03:05,332-0400 DEBUG (jsonrpc/3) [jsonrpc.JsonRpcServer] Calling 'StoragePool.connectStorageServer' in bridge with {u'connectionParams': [{u'id': u'00000000-0000-0000-0000-000000000000', u'connecti
on': u'192.168.200.4', u'iqn': u'iqn.2014-07.org.ovirt:storage', u'user': u'username', u'tpgt': u'1', u'password': '********', u'port': u'3260'}, {u'id': u'00000000-0000-0000-0000-000000000000', u'connection': u
'192.168.201.4', u'iqn': u'iqn.2014-07.org.ovirt:storage', u'user': u'username', u'tpgt': u'1', u'password': '********', u'port': u'3260'}], u'storagepoolID': u'b85207b2-73d6-4b02-818d-d93871e80692', u'domainTyp
e': 3} (__init__:328)
2018-06-06 07:03:05,333-0400 INFO  (jsonrpc/3) [vdsm.api] START connectStorageServer(domType=3, spUUID=u'b85207b2-73d6-4b02-818d-d93871e80692', conList=[{u'id': u'00000000-0000-0000-0000-000000000000', u'connect
ion': u'192.168.200.4', u'iqn': u'iqn.2014-07.org.ovirt:storage', u'user': u'username', u'tpgt': u'1', u'password': '********', u'port': u'3260'}, {u'id': u'00000000-0000-0000-0000-000000000000', u'connection': 
u'192.168.201.4', u'iqn': u'iqn.2014-07.org.ovirt:storage', u'user': u'username', u'tpgt': u'1', u'password': '********', u'port': u'3260'}], options=None) from=::ffff:192.168.201.4,53740, flow_id=2a808cc4, task
_id=902a415b-4a85-488a-9697-d8a12c3ff0f7 (api:47)
2018-06-06 07:03:05,468-0400 DEBUG (jsonrpc/5) [jsonrpc.JsonRpcServer] Calling 'Host.getAllVmStats' in bridge with {} (__init__:328)
2018-06-06 07:03:05,468-0400 INFO  (jsonrpc/5) [api.host] START getAllVmStats() from=::ffff:192.168.201.4,53740 (api:47)
2018-06-06 07:03:05,468-0400 INFO  (jsonrpc/5) [api.host] FINISH getAllVmStats return={'status': {'message': 'Done', 'code': 0}, 'statsList': (suppressed)} from=::ffff:192.168.201.4,53740 (api:53)
2018-06-06 07:03:05,468-0400 DEBUG (jsonrpc/5) [jsonrpc.JsonRpcServer] Return 'Host.getAllVmStats' in bridge with (suppressed) (__init__:355)
2018-06-06 07:03:05,468-0400 INFO  (jsonrpc/5) [jsonrpc.JsonRpcServer] RPC call Host.getAllVmStats succeeded in 0.00 seconds (__init__:311)
2018-06-06 07:03:05,749-0400 DEBUG (jsonrpc/3) [root] /usr/bin/taskset --cpu-list 0-2 /sbin/udevadm settle --timeout=5 (cwd None) (commands:66)
2018-06-06 07:03:05,768-0400 DEBUG (jsonrpc/3) [root] SUCCESS: <err> = ''; <rc> = 0 (commands:87)
2018-06-06 07:03:06,184-0400 DEBUG (jsonrpc/3) [root] /usr/bin/taskset --cpu-list 0-2 /sbin/udevadm settle --timeout=5 (cwd None) (commands:66)
2018-06-06 07:03:06,195-0400 DEBUG (jsonrpc/3) [root] SUCCESS: <err> = ''; <rc> = 0 (commands:87)
2018-06-06 07:03:06,418-0400 DEBUG (jsonrpc/3) [root] /usr/bin/taskset --cpu-list 0-2 /sbin/udevadm settle --timeout=5 (cwd None) (commands:66)
2018-06-06 07:03:06,427-0400 DEBUG (jsonrpc/3) [root] SUCCESS: <err> = ''; <rc> = 0 (commands:87)
2018-06-06 07:03:06,515-0400 INFO  (jsonrpc/3) [vdsm.api] FINISH connectStorageServer return={'statuslist': [{'status': 0, 'id': u'00000000-0000-0000-0000-000000000000'}, {'status': 0, 'id': u'00000000-0000-0000
-0000-000000000000'}]} from=::ffff:192.168.201.4,53740, flow_id=2a808cc4, task_id=902a415b-4a85-488a-9697-d8a12c3ff0f7 (api:53)
2018-06-06 07:03:06,516-0400 DEBUG (jsonrpc/3) [jsonrpc.JsonRpcServer] Return 'StoragePool.connectStorageServer' in bridge with [{'status': 0, 'id': u'00000000-0000-0000-0000-000000000000'}, {'status': 0, 'id': 
u'00000000-0000-0000-0000-000000000000'}] (__init__:355)
2018-06-06 07:03:06,516-0400 INFO  (jsonrpc/3) [jsonrpc.JsonRpcServer] RPC call StoragePool.connectStorageServer succeeded in 1.19 seconds (__init__:311)


Version-Release number of selected component (if applicable):
vdsm-4.30.0-405.git7b1e239.el7.x86_64
ovirt-engine-4.3.0-0.0.master.20180604134534.git3e394fa.el7.noarch
How reproducible:
Always, in O-S-T at least.

Steps to Reproduce:
1. Extend the iSCSI storage domain (with a single LUN).

Additional info:
Command sent to host:
2018-06-06 07:03:05,337-04 INFO  [org.ovirt.engine.core.vdsbroker.vdsbroker.ConnectStorageServerVDSCommand] (default task-2) [2a808cc4] START, ConnectStorageServerVDSCommand(HostName = lago-basic-suite-master-ho
st-1, StorageServerConnectionManagementVDSParameters:{hostId='4d59833e-7b64-46ec-9d98-39104f5e04ec', storagePoolId='b85207b2-73d6-4b02-818d-d93871e80692', storageType='ISCSI', connectionList='[StorageServerConne
ctions:{id='null', connection='192.168.200.4', iqn='iqn.2014-07.org.ovirt:storage', vfsType='null', mountOptions='null', nfsVersion='null', nfsRetrans='null', nfsTimeo='null', iface='null', netIfaceName='null'},
 StorageServerConnections:{id='null', connection='192.168.201.4', iqn='iqn.2014-07.org.ovirt:storage', vfsType='null', mountOptions='null', nfsVersion='null', nfsRetrans='null', nfsTimeo='null', iface='null', ne
tIfaceName='null'}]', sendNetworkEventOnFailure='true'}), log id: 7e0756aa