code: private void checkIfMessageDuplicated(String msgName, Node msgNode) {
      if (messageNames.containsKey(msgName)) {
        MessageLocation location = messageNames.get(msgName);
        compiler.report(JSError.make(msgNode, MESSAGE_DUPLICATE_KEY,
            msgName, location.messageNode.getSourceFileName(),
            Integer.toString(location.messageNode.getLineno())));
      }
    }
docs: Checks if message already processed. If so - it generates 'message
  duplicated' compiler error.
  
  @param msgName the name of the message
  @param msgNode the node that represents JS message