diff --git a/Libraries/Opc.Ua.Server/NodeManager/MasterNodeManager.cs b/Libraries/Opc.Ua.Server/NodeManager/MasterNodeManager.cs
index d826fc35e2..2adbf6aa6a 100644
--- a/Libraries/Opc.Ua.Server/NodeManager/MasterNodeManager.cs
+++ b/Libraries/Opc.Ua.Server/NodeManager/MasterNodeManager.cs
@@ -1198,7 +1198,7 @@ namespace Opc.Ua.Server
         private void PrepareValidationCache<T>(List<T> nodesCollection,
             out Dictionary<NodeId, List<object>> uniqueNodesServiceAttributes)
         {
-            List<NodeId> uniqueNodes = new List<NodeId>();
+            HashSet<NodeId> uniqueNodes = new HashSet<NodeId>();
             for (int i = 0; i < nodesCollection.Count; i++)
             {
                 Type listType = typeof(T);