fix duplicate nodes if node has both ml and data roles

Signed-off-by: Yaliang Wu <ylwu@amazon.com>

plugin/src/main/java/org/opensearch/ml/cluster/DiscoveryNodeHelper.java

@@ -83,7 +83,7 @@ public String[] filterEligibleNodes(String[] nodeIds) {
             return nodeIds;
         }
         DiscoveryNode[] nodes = getNodes(nodeIds);
-        final List<String> eligibleNodes = new ArrayList<>();
+        final Set<String> eligibleNodes = new HashSet<>();
         for (DiscoveryNode node : nodes) {
             if (excludedNodeNames != null && excludedNodeNames.contains(node.getName())) {
                 continue;