there you go
the evil brother (/var/lib/openhab2/config/org/openhab/mqtt-eventbus.config) has gone astray and is sending a second mqtt message (via the EventBus integration which is still active) every time you press that Robo500 button, parallel to the first message that is originating due to the Item config (via the MQTT Binding).
Put the evil brother back in his place by: deleting him 
(effectively, disabling the MQTT-EventBus integration)