Throw an exception if someone tries to register a custom message hand…

…ler twice (it is kind of undefined what should happen in that case, so we just dont!)

locust/runners.py

@@ -432,6 +432,8 @@ def register_message(self, msg_type: str, listener: Callable) -> None:
         :param msg_type: The type of the message to listen for
         :param listener: The function to execute when the message is received
         """
+        if msg_type in self.custom_messages:
+            raise Exception(f"Tried to register listener method for {msg_type}, but it already had a listener!")
         self.custom_messages[msg_type] = listener
 
 

locust/test/test_runners.py

@@ -626,6 +626,20 @@ def on_custom_msg(msg, **kw):
         msg = self.mocked_log.warning[0]
         self.assertIn("Unknown message type received", msg)
 
+    def test_duplicate_message_handler_registration(self):
+        class MyUser(User):
+            @task
+            def my_task(self):
+                pass
+
+        def on_custom_msg(msg, **kw):
+            pass
+
+        environment = Environment(user_classes=[MyUser])
+        runner = LocalRunner(environment)
+        runner.register_message("test_custom_msg", on_custom_msg)
+        self.assertRaises(Exception, runner.register_message, "test_custom_msg", on_custom_msg)
+
     def test_swarm_endpoint_is_non_blocking(self):
         class TestUser1(User):
             @task