If the same event arrives twice (a duplicate), what do you do?

When processing duplicate events, it is important to ensure idempotency — so that reprocessing does not lead to errors or incorrect states.

Approaches:

• Store a unique event identifier (event ID) and check if it has already been processed.
• Use databases or caches (e.g., Redis) to store processed IDs with TTL.
• In the processing logic, ignore events with IDs that have already been encountered.

Example in Go:

var processedEvents = make(map[string]bool)

func handleEvent(eventID string) {
    if processedEvents[eventID] {
        fmt.Println("Duplicate event, skipping")
        return
    }
    // Process the event
    processedEvents[eventID] = true
    fmt.Println("Processing event", eventID)
}

In a real application, it is better to use thread-safe structures and external storage for fault tolerance.