diff --git a/api/src/main/java/be/naaturel/boardmateapi/controllers/ChatController.java b/api/src/main/java/be/naaturel/boardmateapi/controllers/ChatController.java
index 1a1151ce..dcf354c0 100644
--- a/api/src/main/java/be/naaturel/boardmateapi/controllers/ChatController.java
+++ b/api/src/main/java/be/naaturel/boardmateapi/controllers/ChatController.java
@@ -17,7 +17,9 @@ import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.*;
 
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 
 @RestController
@@ -26,7 +28,7 @@ public class ChatController {
     private final MessageService messageService;
     private final WebexService webexService;
     private final MqttService mqttService;
-    private final ConcurrentHashMap<String, Integer> expectedMessages;
+    private final Set<String> expectedMessages;
 
     @Autowired
     public ChatController(
@@ -36,7 +38,7 @@ public class ChatController {
         this.messageService = messageService;
         this.webexService = webexService;
         this.mqttService = mqttService;
-        this.expectedMessages = new ConcurrentHashMap<>();
+        this.expectedMessages = ConcurrentHashMap.newKeySet();
     }
 
     @PostMapping("/message/send")
@@ -108,10 +110,11 @@ public class ChatController {
 
             ObjectMapper mapper = new ObjectMapper();
             WebexWebhook payload = mapper.readValue(rawPayload, WebexWebhook.class);
+            String messageId = payload.getData().getId();
 
-            boolean firstTime = expectedMessages.putIfAbsent(payload.getData().getId(), 1) == null;
+            boolean firstTime = expectedMessages.add(messageId);
             if (!firstTime) {
-                Logger.displayInfo("Rejected message: " + payload.getData().getId());
+                Logger.displayInfo("Duplicate or already processed → Rejected: " + messageId);
                 return ResponseEntity.status(HttpStatus.UNAUTHORIZED).body(result);
             }