docs: private void checkNoDuplicateKeyAttribute(KeyNode node) {
      SoyNode parentNode = node.getParent();
      if (!(parentNode instanceof HtmlOpenTagNode)) {
        // Error thrown before this.
        return;
      }
  
      HtmlAttributeNode keyAttrNode = ((HtmlOpenTagNode) parentNode).getDirectAttributeNamed("key");
      if (keyAttrNode != null) {
        errorReporter.report(keyAttrNode.getSourceLocation(), DUPLICATE_KEY_ATTR);
      }
    }