Fix duplicate attribute removal in WP_HTML_Template

When using false/null to remove a placeholder attribute, duplicate
attributes (e.g., `<input disabled="</%d>" disabled>`) now get removed
as well.

The fix pre-computes duplicate removal edits in compile(), following
the existing pattern for text normalization and attribute escaping.
This is more efficient than waiting for normalize() to strip them at
render time, and correctly handles the case where removing the first
attribute would leave the duplicate as the primary attribute.

Changes:
- Make duplicate_attributes protected in WP_HTML_Tag_Processor
- Expose get_duplicate_attributes() in Template's anonymous class
- Emit removal edits for all duplicates in compile()

Author: sirreal

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

@@ -718,7 +718,7 @@ class WP_HTML_Tag_Processor {
 	 *
 	 * @var (WP_HTML_Span[])[]|null
 	 */
-	private $duplicate_attributes = null;
+	protected $duplicate_attributes = null;
 
 	/**
 	 * Which class names to add or remove from a tag.

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

@@ -142,6 +142,10 @@ public function get_tag_attributes(): array {
 			public function create_fragment_at_node( string $html ): ?static {
 				return $this->create_fragment_at_current_node( $html );
 			}
+
+			public function get_duplicate_attributes(): ?array {
+				return $this->duplicate_attributes;
+			}
 		} )::create_fragment( $this->template_string );
 
 		if ( null === $processor ) {
@@ -314,6 +318,20 @@ public function create_fragment_at_node( string $html ): ?static {
 							}
 						}
 					}
+
+					// Remove duplicate attributes (invalid HTML, stripped during normalization).
+					$duplicates = $processor->get_duplicate_attributes();
+					if ( null !== $duplicates ) {
+						foreach ( $duplicates as $spans ) {
+							foreach ( $spans as $span ) {
+								$this->edits[] = new WP_HTML_Text_Replacement(
+									$span->start,
+									$span->length,
+									'',
+								);
+							}
+						}
+					}
 					break;
 			}
 		}