How did the two messages appear? Why did the same one get into the handler twice?

Two identical messages could have appeared due to a repeated event submission or a logic error in processing. For example, if the handler is not disabled after the first trigger, or if the event is generated multiple times in a row (for instance, due to network delays or repeated calls).

It is also possible that the system did not mark the message as processed, and it was processed again (for example, during re-delivery in a message queue).

To prevent this, the following are usually applied:

• idempotency of handlers (so that repeated calls do not lead to errors);
• marking processed messages;
• disabling handlers after the first trigger, if necessary;
• using debounce/throttle to control call frequency.