JMRI keeps repeating MQTT messages to my Pico Wifi via Hivemq. Is there a way to stop this or reduce the repeitition?

Question

I borrowed code from Toms Hardware on how to use MQTT and subscribe. JRMI is the publisher of the messages and it keeps repeating them over and over again. Is there anyway to have the message sent only once? I dont have this problem when I subscribe to MQTT via http://www.hivemq.com/demos/websocket-client/ The MQTT service I'm using is broker.hivemq.com

For those not familiar with JRMI, it is the JAVA program that model railroads use to control tracks,lighting, DCC etc. Ref: https://www.jmri.org/

The link to Tom's is here https://www.tomshardware.com/how-to/send-and-receive-data-raspberry-pi-pico-w-mqtt

The code adapted from Tom's is

import network
import time
from machine import Pin
from umqtt.simple import MQTTClient

wlan = network.WLAN(network.STA_IF)
wlan.active(True)
wlan.connect("whatever","pwd")
time.sleep(5)
print(wlan.isconnected())



mqtt_server = 'broker.hivemq.com'
client_id = 'bigles'
topic_sub = b'/trains/track/turnout/#'

def sub_cb(topic, msg):
    print("New message on topic {}".format(topic.decode('utf-8')))
    msg = msg.decode('utf-8')
    print(msg)
def mqtt_connect():
    client = MQTTClient(client_id, mqtt_server, keepalive=60)
    client.set_callback(sub_cb)
    client.connect()
    print('Connected to %s MQTT Broker'%(mqtt_server))
    return client

def reconnect():
    print('Failed to connect to MQTT Broker. Reconnecting...')
    time.sleep(5)
    machine.reset()
    
try:
    client = mqtt_connect()
except OSError as e:
    reconnect()
while True:
    client.subscribe(topic_sub)
    time.sleep(1)

The setup inside JRMI for MQTT (edit->preferences) is as follows:

Answer 1

JMRI, by default, publishes with "the retain option on". When you subscribe to a topic the broker will send you the most recent (if any) retained message. This occurs even if you already had an identical subscription as per the MQTT Spec:

If a Server receives a SUBSCRIBE Packet containing a Topic Filter that is identical to an existing Subscription’s Topic Filter then it MUST completely replace that existing Subscription with a new Subscription. The Topic Filter in the new Subscription will be identical to that in the previous Subscription, although its maximum QoS value could be different. Any existing retained messages matching the Topic Filter MUST be re-sent, but the flow of publications MUST NOT be interrupted [MQTT-3.8.4-3].

In your code you are calling Subscribe in a loop:

while True:
    client.subscribe(topic_sub)
    time.sleep(1)

To avoid the repeated messages move the subscribe out of the loop (you only need to subscribe once!). Something like the following (simplified!) code:

client = mqtt_connect()
client.subscribe(topic_sub)

while True:
    client.wait_msg()   // Use client.check_msg() if you have other stuff to do