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Subject: [OASIS Issue Tracker] Commented: (MQTT-59) How are duplicate SUBSCRIBEs handled?

    [ http://tools.oasis-open.org/issues/browse/MQTT-59?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=34211#action_34211 ] 

Nick O'Leary commented on MQTT-59:

If a client sends a SUBSCRIBE to a topic it is already subscribed to, this new subscription REPLACES the existing one - and the new qos value is applied.

The end result is the client still has a single subscription to the topic and will receive one copy of any message.

If the topic has any retained messages, they will be resent for each SUBSCRIBE received.

There is another scenario to consider: if a client sends a single SUBSCRIBE control packet with multiple instances of the same topic in it. I suspect existing implementations will process each topic in the order they are specified in the SUBSCRIBE packet and if there is a duplicate, the last one processed will be the one that 'wins'.

> How are duplicate SUBSCRIBEs handled?
> -------------------------------------
>                 Key: MQTT-59
>                 URL: http://tools.oasis-open.org/issues/browse/MQTT-59
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Improvement
>          Components: core
>            Reporter: Allan Stockdill-Mander 
> This issue is related to MQTT-58
> If a client has a subscription to a topic and then issues a SUBSCRIBE to the same topic does it have two subscriptions (and depending on MQTT-58 receive two copies of each message) or just one? If the latter case do the values of the new SUBSCRIBE (such as QoS) replace those of the original subscription?

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