Prevent printing duplicate attributes after normalizing the attribute names.

Author: dmsnell

src/wp-includes/html-api/class-wp-html-processor.php

@@ -1417,8 +1417,16 @@ public function serialize_token(): string {
 		$html .= "<{$qualified_name}";
 
 		$previous_attribute_was_true = false;
+		$seen_attribute_names        = array();
 		foreach ( $attribute_names as $attribute_name ) {
 			$qualified_attribute_name = $this->get_qualified_attribute_name( $attribute_name );
+			$qualified_attribute_name = str_replace( "\x00", "\u{FFFD}", $qualified_attribute_name );
+			$qualified_attribute_name = wp_scrub_utf8( $qualified_attribute_name );
+			if ( isset( $seen_attribute_names[ $qualified_attribute_name ] ) ) {
+				continue;
+			} else {
+				$seen_attribute_names[ $qualified_attribute_name ] = true;
+			}
 
 			if (
 				$previous_attribute_was_true &&