code: @Override
    public void addListener(INetworkConnectionListener listener) {
      if (!this.listeners.contains(listener)) {
        this.listeners.add(listener);
      }
    }
docs: /*
  (non-Javadoc)
  
  @see org.jdiameter.server.api.io.INetworkGuard#addListener(org.jdiameter.server .api.io.INetworkConnectionListener)