When an actor subscribes to overlapping topics, e.g. a/+ and +/b, then all messages received on the overlapping topics (in that case a/b) are forwarded twice to the actor, and even four times with some MQTT brokers (rumqttc and nanomq).

• Little can be done when the broker itself duplicates the messages. But at least, the tedge_mqtt_ext layer should not add its own set of duplicates.
• Its difficult to avoid such overlapping subscriptions with tedge flows where the subscriptions are independently configured for each flow.

Proposed changes

• Let tedge_mqtt_ext removes duplicates in the list of clients for a message

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s. You can activate automatic signing by running just prepare-dev once)
• I ran just format as mentioned in CODING_GUIDELINES
• I used just check as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments