code: private boolean updateDelivered(Message message) {
          if (consumers.size() <= 1) {
              return true;
          }
          
          String pubid = (String) message.getMetadata("cwf.pub.event");
          return deliveredMessageCache.putIfAbsent(pubid, "") == null;
      }
docs: Updates the delivered message cache. This avoids delivering the same message transported by
  different messaging frameworks. If we have only one consumer registered, we don't need to
  worry about this.
  
  @param message The message being delivered.
  @return True if the cache was updated (i.e., the message has not been previously delivered).