MQTT QoS 1 Messages Duplicated

In this tutorial, you'll learn about MQTT QoS 1 Messages Duplicated. We cover key concepts, practical examples, and best practices.

The Problem

QoS 1 messages are delivered more than once to the subscriber.

Quick Fix

Wrong

client.publish('cmd', payload, { qos: 1 })  # At least once

Subscriber receives the same message 2+ times.

Right

// Add deduplication in subscriber
const processedIds = new Set()

client.on('message', (topic, payload, packet) => {
  // MQTT 5.0 topic alias or message expiry
  // Use message ID for dedup (QoS 1 has Packet ID)
  if (packet.qos === 1) {
    const msgId = packet.messageId || packet.properties?.messageExpiryInterval
    
    if (processedIds.has(msgId)) {
      return  // Skip duplicate
    }
    processedIds.add(msgId)
  }
  
  console.log('Processing:', topic, payload.toString())
})

// Or use QoS 2 for exactly-once delivery
client.publish('cmd', payload, { qos: 2 })

Messages delivered once (deduplicated or QoS 2).

Prevention

QoS 1 = at least once. Broker stores the message and retries until PUBACK received. The subscriber may get duplicates if PUBACK is lost. Solutions: implement deduplication, use idempotent handlers, or use QoS 2 for exactly-once delivery. QoS 1 message IDs are stored in session for offline queuing.

DodaTech engineers apply these same patterns across Doda Browser, DodaZIP, and Durga Antivirus Pro for production IoT reliability.

FAQ