QOS 2 can deliver a message twice

[daisukes]

Hello,

I have experienced an issue where my app delivers a message twice when the app has a disconnection/reconnection, even if I set the message QOS to 2. I want to send a message every second and deliver all the messages even if the phone is disconnected for a while.

If I set cleanSession to True, it looks like the app cleans the queue and loses some messages. If I set cleanSession to False, I got the issue.

I found that recoverSessionBy enqueues stored messages and can duplicate messages. I think it should check if the mqueue already holds the same message when a stored message is appended.

https://github.com/emqx/CocoaMQTT/blob/ff7c07b8a212e2e724d93a91d165a899df546db0/Source/CocoaMQTTDeliver.swift#L78-L94

The way to reproduce the issue

• make an instance with cleanSession = false

  self.mqtt = CocoaMQTT(clientID: clientID, host: host, port: port)
  mqtt.cleanSession = false

• connect to the broker via Wi-Fi
• record delivered messages by subscribing to the message (I used Mosquitto as the broker and subscribed by a client with paho.mqtt.cpp)
• publish a message every second (keeps publishing while it is disconnected)
• disconnect intentionally by disabling Wi-Fi, wait 10 seconds, and enable Wi-Fi
• wait until the connection is recovered and it sends the all messages
• stop

Behavior

• My client received all the messages + duplicated messages published while the app was disconnected.

Expected

• My client received all the messages without any duplication

Thank you!