Implement R010: Add duplicate message handling

- Added duplicate message ID tracking with 1-hour retention
- Messages with same ID are rejected within the prevention window
- Added cleanup for expired message IDs
- Added comprehensive unit tests for duplicate handling

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: stho32

Source/LocalNetAppChat/LocalNetAppChat.Server.Domain.Tests/Messaging/MessageListTests.cs

@@ -77,4 +77,84 @@ public void We_can_get_a_little_status_report_from_it()
         var messageList = GetMessageList();
         Assert.IsNotEmpty(messageList.GetStatus());
     }
+
+    [Test]
+    public void Duplicate_messages_with_same_id_are_rejected()
+    {
+        var messageList = GetMessageList();
+        var messageId = Guid.NewGuid().ToString();
+        
+        // Create two messages with the same ID
+        var message1 = CreateMessageWithId(messageId);
+        var message2 = CreateMessageWithId(messageId);
+        
+        // Add first message
+        messageList.Add(message1);
+        
+        // Try to add duplicate
+        messageList.Add(message2);
+        
+        // Should only have one message
+        var messagesForClient = messageList.GetMessagesForClient("TestClient");
+        Assert.AreEqual(1, messagesForClient.Length);
+        Assert.AreEqual(messageId, messagesForClient[0].Message.Id);
+    }
+
+    [Test]
+    public void Duplicate_messages_are_rejected_within_one_hour()
+    {
+        var messageList = GetMessageList();
+        var messageId = Guid.NewGuid().ToString();
+        
+        // Add first message
+        var message1 = CreateMessageWithId(messageId);
+        messageList.Add(message1);
+        
+        // Try to add another message with same ID within the hour
+        var message2 = CreateMessageWithId(messageId);
+        messageList.Add(message2);
+        
+        // Should still only have one message
+        var messagesForClient = messageList.GetMessagesForClient("TestClient");
+        Assert.AreEqual(1, messagesForClient.Length);
+        
+        // Try with a third message
+        var message3 = CreateMessageWithId(messageId);
+        messageList.Add(message3);
+        
+        // Clear client state and check again
+        messageList.GetMessagesForClient("TestClient");
+        var allMessages = messageList.GetMessagesForClient("TestClient");
+        Assert.AreEqual(0, allMessages.Length); // 0 because we already retrieved it
+    }
+
+    [Test]
+    public void Messages_without_id_are_always_accepted()
+    {
+        var messageList = GetMessageList();
+        
+        // Create messages without ID (empty string)
+        var message1 = CreateMessageWithId("");
+        var message2 = CreateMessageWithId("");
+        
+        messageList.Add(message1);
+        messageList.Add(message2);
+        
+        var messagesForClient = messageList.GetMessagesForClient("TestClient");
+        Assert.AreEqual(2, messagesForClient.Length);
+    }
+
+    private static ReceivedMessage CreateMessageWithId(string messageId, DateTime? explicitTime = null)
+    {
+        var processors = MessageProcessorFactory.Get(
+            new ThreadSafeCounter(),
+            new DateTimeProviderMock(explicitTime ?? DateTime.Now));
+
+        var message = new LnacMessage(messageId, "TestSender", "Test Message",
+            Array.Empty<string>(),
+            true,
+            "Message").ToReceivedMessage();
+
+        return processors.Process(message);
+    }
 }

Source/LocalNetAppChat/LocalNetAppChat.Server.Domain/SynchronizedCollectionBasedMessageList.cs

@@ -10,6 +10,8 @@ public class SynchronizedCollectionBasedMessageList : IMessageList
     private readonly SynchronizedCollection<ReceivedMessage> _messages = new();
     private readonly ConcurrentDictionary<long, long> _redirectedPatternMessages = new();
     private readonly ConcurrentDictionary<string, long> _clientStates = new();
+    private readonly ConcurrentDictionary<string, DateTime> _messageIdTracker = new();
+    private readonly TimeSpan _duplicatePreventionLifetime = TimeSpan.FromHours(1);
 
     public SynchronizedCollectionBasedMessageList(TimeSpan messageLifetime)
     {
@@ -19,6 +21,7 @@ public SynchronizedCollectionBasedMessageList(TimeSpan messageLifetime)
     private void Cleanup()
     {
         var currentEndOfLife = CurrentEndOfLife();
+        var duplicateTrackerEndOfLife = DateTime.Now - _duplicatePreventionLifetime;
 
         for (var i = _messages.Count-1; i >= 0; i--)
         {
@@ -31,11 +34,40 @@ private void Cleanup()
                     _redirectedPatternMessages.TryRemove(message.Id, out _);
             }
         }
+
+        // Clean up expired message IDs from duplicate tracker
+        var expiredIds = _messageIdTracker
+            .Where(kvp => kvp.Value < duplicateTrackerEndOfLife)
+            .Select(kvp => kvp.Key)
+            .ToList();
+
+        foreach (var id in expiredIds)
+        {
+            _messageIdTracker.TryRemove(id, out _);
+        }
     }
 
 
     public void Add(ReceivedMessage receivedMessage)
     {
+        // Check if this message ID was already received within the duplicate prevention lifetime
+        if (!string.IsNullOrEmpty(receivedMessage.Message.Id))
+        {
+            var now = DateTime.Now;
+            if (_messageIdTracker.TryGetValue(receivedMessage.Message.Id, out var existingTimestamp))
+            {
+                // Message with this ID already exists and hasn't expired
+                if (existingTimestamp > now - _duplicatePreventionLifetime)
+                {
+                    // Reject duplicate message
+                    return;
+                }
+            }
+            
+            // Track this message ID
+            _messageIdTracker.AddOrUpdate(receivedMessage.Message.Id, now, (_, _) => now);
+        }
+
         _messages.Add(receivedMessage);
 
         Cleanup();