diff --git a/php-transformer/src/HtmlToBlocks/BlockFactory.php b/php-transformer/src/HtmlToBlocks/BlockFactory.php
index 3f160525..5553e608 100644
--- a/php-transformer/src/HtmlToBlocks/BlockFactory.php
+++ b/php-transformer/src/HtmlToBlocks/BlockFactory.php
@@ -62,6 +62,8 @@ public function create(string $name, array $attrs = array(), array $innerBlocks
      */
     private function normalizeAttrsForBlock(string $name, array $attrs): array
     {
+        $attrs = $this->normalizeClassNameAttr($attrs);
+
         if ( in_array($name, array( 'core/group', 'core/columns' ), true) ) {
             unset($attrs['style']['dimensions']['maxWidth']);
             if ( empty($attrs['style']['dimensions']) ) {
@@ -85,6 +87,33 @@ private function normalizeAttrsForBlock(string $name, array $attrs): array
         return $attrs;
     }
 
+    /**
+     * @param array<string, mixed> $attrs
+     * @return array<string, mixed>
+     */
+    private function normalizeClassNameAttr(array $attrs): array
+    {
+        if ( ! is_string($attrs['className'] ?? null) ) {
+            return $attrs;
+        }
+
+        $classes = array();
+        foreach ( preg_split('/\s+/', trim($attrs['className'])) ?: array() as $class ) {
+            if ( '' === $class || GeneratedGutenbergClassPolicy::isGeneratedClassName($class) || in_array($class, $classes, true) ) {
+                continue;
+            }
+            $classes[] = $class;
+        }
+
+        if ( array() === $classes ) {
+            unset($attrs['className']);
+            return $attrs;
+        }
+
+        $attrs['className'] = implode(' ', $classes);
+        return $attrs;
+    }
+
     /**
      * @param array<string, mixed> $attrs
      * @return array<string, mixed>
diff --git a/php-transformer/tests/contract/run.php b/php-transformer/tests/contract/run.php
index 356faebb..c8d3a4b8 100644
--- a/php-transformer/tests/contract/run.php
+++ b/php-transformer/tests/contract/run.php
@@ -804,6 +804,24 @@ public function match(DOMElement $element, PatternContext $context): ?array
 $assert(str_contains($canonicalWrapperAttrsSerialized, '<h2 class="wp-block-heading has-text-color section-title" style="color:red">Menu</h2>'), 'heading wrappers include canonical and support classes with supported color style');
 $assert(str_contains($canonicalWrapperAttrsSerialized, '<p class="card-desc" style="margin-bottom:1rem">Fresh daily.</p>'), 'paragraph wrappers preserve runtime-addressable classes and supported margin style');
 
+$paragraphGeneratedClassLeakResult = ( new HtmlTransformer() )->transform(
+    '<main><p class="has-text-color has-text-color hero-tagline" style="color:red">Slow roasted daily.</p></main>'
+)->toArray();
+$paragraphGeneratedClassLeakBlock = $paragraphGeneratedClassLeakResult['blocks'][0] ?? array();
+$paragraphGeneratedClassLeakSerialized = (string) ($paragraphGeneratedClassLeakResult['serialized_blocks'] ?? '');
+$assert('hero-tagline' === ($paragraphGeneratedClassLeakBlock['attrs']['className'] ?? ''), 'paragraph className strips duplicate generated color classes while preserving custom classes');
+$assert(str_contains($paragraphGeneratedClassLeakSerialized, '<p class="has-text-color hero-tagline" style="color:red">Slow roasted daily.</p>'), 'paragraph serialization emits generated has-text-color once before custom classes');
+$assert(1 === substr_count($paragraphGeneratedClassLeakSerialized, 'has-text-color'), 'paragraph serialization emits has-text-color exactly once');
+$assert('pass' === ($paragraphGeneratedClassLeakResult['source_reports']['wp_block_validity']['status'] ?? ''), 'paragraph with stripped generated className passes generated block validity checks');
+
+$paragraphFactoryGeneratedClassLeakBlock = ( new BlockFactory() )->create('core/paragraph', array(
+    'content'   => 'Slow roasted daily.',
+    'className' => 'has-text-color has-text-color hero-tagline',
+    'style'     => array('color' => array('text' => 'red')),
+));
+$assert('hero-tagline' === ($paragraphFactoryGeneratedClassLeakBlock['attrs']['className'] ?? ''), 'BlockFactory strips generated classes from direct paragraph className attrs');
+$assert(str_contains((string) ($paragraphFactoryGeneratedClassLeakBlock['innerHTML'] ?? ''), '<p class="has-text-color hero-tagline" style="color:red">Slow roasted daily.</p>'), 'BlockFactory paragraph innerHTML emits a single generated color class');
+
 $paragraphSvgResult = ( new HtmlTransformer() )->transform(
     '<main><p class="social-link"><a class="social-link" href="#" aria-label="Follow"><svg viewBox="0 0 10 10" aria-hidden="true"><path d="M0 0h10v10H0z"></path></svg></a></p></main>'
 )->toArray();