code: func (sp *ServerPeer) addKnownAddresses(addresses []*wire.NetAddress) {
  	for _, na := range addresses {
  		sp.knownAddresses[addrmgr.NetAddressKey(na)] = struct{}{}
  	}
  }
docs: // addKnownAddresses adds the given addresses to the set of known addresses to
  // the peer to prevent sending duplicate addresses.