Running ascli I get a lot of 2016-07-14 15:44:07 WARN AEROSPIKE_ERR_TIMEOUT warnings. I see that in asadm -> info network there are 13 nodes listed, but only 4 are active (which is correct; it's a 4 node cluster). Those other 9 nodes are either old hosts which have left the cluster ages ago, or other IP address for hosts which are (correctly) part of the cluster. The cluster works fine otherwise. All nodes run 3.7.4.1.
Is there a way to remove all those old hosts? dunning them doesn't do anything.
You are looking for the services-alumni-reset command. Basically Aerospike doesn't know you do not intend to return these nodes in the future so it keeps these entries in its internal alumni list to allow tools to detect nodes that are missing from the cluster. The reset commands syncs the alumni list with the active node list and may be used whenever a node has been permanently removed from the cluster.
NB, the alumni list doesn't affect the Aerospike servers or clients, it is provided specifically for troubleshooting and used by various diagnostic tools.