code: @Override
      public void addHierarchyEventListener(HierarchyEventListener listener) {
          if (listeners.contains(listener)) {
              LogLog.warn("Ignoring attempt to add an existent listener.");
          } else {
              listeners.add(listener);
          }
      }
docs: /*
  (non-Javadoc)
  
  @see
  org.apache.log4j.Hierarchy#addHierarchyEventListener(org.apache.log4j
  .spi.HierarchyEventListener)