Fix race condition causing duplicate MonitoredItems in concurrent CreateItemsAsync calls

Libraries/Opc.Ua.Client/Subscription/MonitoredItemStatus.cs

@@ -46,6 +46,12 @@ public sealed record class MonitoredItemStatus
         /// </summary>
         public bool Created => Id != 0;
 
+        /// <summary>
+        /// Whether a create request has been sent to the server but not yet completed.
+        /// This flag is used to prevent duplicate creation requests in multi-threaded scenarios.
+        /// </summary>
+        internal bool Creating { get; set; }
+
         /// <summary>
         /// Any error condition associated with the monitored item.
         /// </summary>
@@ -170,6 +176,9 @@ internal void SetCreateResult(
                     FilterResult = Utils.Clone(result.FilterResult.Body) as MonitoringFilterResult;
                 }
             }
+
+            // Clear the Creating flag now that the create operation is complete
+            Creating = false;
         }
 
         /// <summary>

Libraries/Opc.Ua.Client/Subscription/Subscription.cs

@@ -1014,25 +1014,37 @@ public async Task<IList<MonitoredItem>> CreateItemsAsync(CancellationToken ct =
             }
 
             using Activity? activity = m_telemetry.StartActivity();
-            // create monitored items.
-            CreateMonitoredItemsResponse response = await Session
-                .CreateMonitoredItemsAsync(null, Id, TimestampsToReturn, requestItems, ct)
-                .ConfigureAwait(false);
+            try
+            {
+                // create monitored items.
+                CreateMonitoredItemsResponse response = await Session
+                    .CreateMonitoredItemsAsync(null, Id, TimestampsToReturn, requestItems, ct)
+                    .ConfigureAwait(false);
 
-            MonitoredItemCreateResultCollection results = response.Results;
-            ClientBase.ValidateResponse(results, itemsToCreate);
-            ClientBase.ValidateDiagnosticInfos(response.DiagnosticInfos, itemsToCreate);
+                MonitoredItemCreateResultCollection results = response.Results;
+                ClientBase.ValidateResponse(results, itemsToCreate);
+                ClientBase.ValidateDiagnosticInfos(response.DiagnosticInfos, itemsToCreate);
 
-            // update results.
-            for (int ii = 0; ii < results.Count; ii++)
+                // update results.
+                for (int ii = 0; ii < results.Count; ii++)
+                {
+                    itemsToCreate[ii]
+                        .SetCreateResult(
+                            requestItems[ii],
+                            results[ii],
+                            ii,
+                            response.DiagnosticInfos,
+                            response.ResponseHeader);
+                }
+            }
+            catch
             {
-                itemsToCreate[ii]
-                    .SetCreateResult(
-                        requestItems[ii],
-                        results[ii],
-                        ii,
-                        response.DiagnosticInfos,
-                        response.ResponseHeader);
+                // Clear the Creating flag on all items if an exception occurs
+                foreach (MonitoredItem monitoredItem in itemsToCreate)
+                {
+                    monitoredItem.Status.Creating = false;
+                }
+                throw;
             }
 
             m_changeMask |= SubscriptionChangeMask.ItemsCreated;
@@ -2795,8 +2807,8 @@ private static bool UpdateMonitoringMode(
             {
                 foreach (MonitoredItem monitoredItem in m_monitoredItems.Values)
                 {
-                    // ignore items that have been created.
-                    if (monitoredItem.Status.Created)
+                    // ignore items that have been created or are being created.
+                    if (monitoredItem.Status.Created || monitoredItem.Status.Creating)
                     {
                         continue;
                     }
@@ -2825,6 +2837,13 @@ private static bool UpdateMonitoringMode(
                     requestItems.Add(request);
                     itemsToCreate.Add(monitoredItem);
                 }
+
+                // Mark all items as being created before releasing the lock
+                // to prevent duplicate creation requests in multi-threaded scenarios
+                foreach (MonitoredItem monitoredItem in itemsToCreate)
+                {
+                    monitoredItem.Status.Creating = true;
+                }
             }
             return (requestItems, itemsToCreate);
         }

Tests/Opc.Ua.Client.Tests/SubscriptionTest.cs

@@ -1556,5 +1556,78 @@ public async Task SetTriggeringTrackingAsync()
             // Clean up
             await subscription.DeleteAsync(true, CancellationToken.None).ConfigureAwait(false);
         }
+
+        /// <summary>
+        /// Test that concurrent calls to CreateItemsAsync do not create duplicate monitored items.
+        /// This test verifies the fix for the race condition where multiple threads calling
+        /// CreateItemsAsync could include the same items in their create requests.
+        /// </summary>
+        [Test]
+        [Order(1100)]
+        public async Task ConcurrentCreateItemsNoDuplicates()
+        {
+            var subscription = new TestableSubscription(Session.DefaultSubscription);
+            Session.AddSubscription(subscription);
+            await subscription.CreateAsync().ConfigureAwait(false);
+
+            // Create multiple monitored items
+            var items = new List<MonitoredItem>();
+            for (int i = 0; i < 10; i++)
+            {
+                items.Add(new TestableMonitoredItem(subscription.DefaultItem)
+                {
+                    DisplayName = $"Item{i}",
+                    StartNodeId = VariableIds.Server_ServerStatus_CurrentTime,
+                    AttributeId = Attributes.Value
+                });
+            }
+
+            subscription.AddItems(items);
+            Assert.That(subscription.MonitoredItemCount, Is.EqualTo(10));
+
+            // Simulate concurrent CreateItemsAsync calls
+            // Use 3 concurrent tasks to ensure at least 2 will race with each other
+            const int ConcurrentTasks = 3;
+            var tasks = new List<Task<IList<MonitoredItem>>>();
+            for (int i = 0; i < ConcurrentTasks; i++)
+            {
+                tasks.Add(Task.Run(() =>
+                    subscription.CreateItemsAsync(CancellationToken.None)));
+            }
+
+            var results = await Task.WhenAll(tasks).ConfigureAwait(false);
+
+            // Verify that all items were created exactly once
+            int totalCreated = 0;
+            foreach (var item in items)
+            {
+                if (item.Status.Created)
+                {
+                    totalCreated++;
+                    Assert.That(item.Status.Id, Is.GreaterThan(0u), 
+                        $"Item {item.DisplayName} should have a server-assigned ID");
+                }
+            }
+
+            Assert.That(totalCreated, Is.EqualTo(10), 
+                "All 10 items should be created exactly once");
+
+            // Verify that each result list contains only the items that were actually created
+            // by that specific call (should be empty for concurrent calls after the first)
+            int nonEmptyResults = 0;
+            foreach (var result in results)
+            {
+                if (result.Count > 0)
+                {
+                    nonEmptyResults++;
+                }
+            }
+
+            Assert.That(nonEmptyResults, Is.LessThanOrEqualTo(1),
+                "Only one CreateItemsAsync call should have created items");
+
+            // Clean up
+            await subscription.DeleteAsync(true, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }