The load balancing feature supports the following:

The Broker load-balancing feature does not support by-publisher-order delivery (that is, there is no guarantee that messages will arrive at a subscriber in the same order in which they were sent by the publisher). Once-and-only once delivery is not guaranteed for the multisend best effort and multisend guaranteed policies if any of the Broker goes down during publishing or subscription. For more information about multisend best effort and multisend guaranteed policies, see Cluster Load Balancing Policies.

You need to add your own custom application code to guarantee once-and-only-once message delivery.