Issue #2945033 by longwave, yobottehg, TMWagner, vdsh, jlatorre, catch:...

Issue #2945033 by longwave, yobottehg, TMWagner, vdsh, jlatorre, catch: HtmlHeadLink processing does not allow for duplicated alternate hreflang links

core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php

@@ -436,7 +436,15 @@ protected function processHtmlHeadLink(array $html_head_link) {
         '#attributes' => $attributes,
       ];
       $href = $attributes['href'];
-      $attached['html_head'][] = [$element, 'html_head_link:' . $attributes['rel'] . ':' . $href];
+      $rel = $attributes['rel'];
+
+      // Allow multiple hreflang tags to use the same href.
+      if (isset($attributes['hreflang'])) {
+        $attached['html_head'][] = [$element, 'html_head_link:' . $rel . ':' . $attributes['hreflang'] . ':' . $href];
+      }
+      else {
+        $attached['html_head'][] = [$element, 'html_head_link:' . $rel . ':' . $href];
+      }
 
       if ($should_add_header) {
         // Also add a HTTP header "Link:".

core/modules/system/tests/modules/render_attached_test/src/Controller/RenderAttachedTestController.php

@@ -80,6 +80,7 @@ public function htmlHeaderLink() {
     $render['#attached']['html_head_link'][] = [['href' => '/foo?bar=<baz>&baz=false', 'rel' => 'alternate'], TRUE];
     $render['#attached']['html_head_link'][] = [['href' => '/not-added-to-http-headers', 'rel' => 'alternate'], FALSE];
     $render['#attached']['html_head_link'][] = [['href' => '/foo/bar', 'hreflang' => 'nl', 'rel' => 'alternate'], TRUE];
+    $render['#attached']['html_head_link'][] = [['href' => '/foo/bar', 'hreflang' => 'de', 'rel' => 'alternate'], TRUE];
     return $render;
   }
 

core/modules/system/tests/src/Functional/Render/HtmlResponseAttachmentsTest.php

@@ -65,8 +65,13 @@ public function testAttachments() {
     $expected_link_headers = [
       '</foo?bar=&lt;baz&gt;&amp;baz=false>; rel="alternate"',
       '</foo/bar>; hreflang="nl"; rel="alternate"',
+      '</foo/bar>; hreflang="de"; rel="alternate"',
     ];
     $this->assertEquals($expected_link_headers, $this->getSession()->getResponseHeaders()['Link']);
+
+    // Check that duplicate alternate URLs with different hreflangs are allowed.
+    $test_link = $this->xpath('//head/link[@rel="alternate"][@href="/foo/bar"]');
+    $this->assertEquals(2, count($test_link), 'Duplicate alternate URLs are allowed.');
   }
 
   /**