private List<RemoteInstance> distinct(List<RemoteInstance> instanceList) {
        Set<Address> addresses = new HashSet<>();
        List<RemoteInstance> newInstanceList = new ArrayList<>();
        instanceList.forEach(instance -> {
            if (addresses.add(instance.getAddress())) {
                newInstanceList.add(instance);
            }
        });
        return newInstanceList;
    }
Because of OAP server register by the UUID which one-to-one mapping with process number. The register information
not delete immediately after process shutdown because of there is always happened network fault, not really
process shutdown. So, cluster module must wait a few seconds to confirm it. Then there are more than one register
information in the cluster.

@param instanceList the instances query from cluster module.
@return distinct remote instances
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/remote/client/RemoteClientManager.java