code: protected boolean isDuplicate(String eventID) {
          if (this.receivedEvents == null) {
              this.receivedEvents = new LRUCache<String>();
          }
  
          return !this.receivedEvents.add(eventID);
      }
docs: Indicates whether or not an event ID is a duplicate.
  
  <p>This method can be overridden by a subclass in order to provide custom de-duping logic.</p>
  
  @param  eventID the event ID.
  @return         true if the event is a duplicate; otherwise false.