code: @Override
      public void notifyMessageReceived(SQSMessage message) throws JMSException {
          SQSMessageIdentifier messageIdentifier = SQSMessageIdentifier.fromSQSMessage(message);
          if (!unAckMessages.contains(messageIdentifier)) {
              unAckMessages.add(messageIdentifier);
          }
      }
docs: Updates the internal queue for the consumed but not acknowledged
  messages if the message was not already on queue.