core/modules/comment/src/CommentViewBuilder.php                |  9 ++++++---
 core/modules/comment/src/Tests/CommentCacheTagsTest.php        | 10 ++++++++--
 .../comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php |  5 +----
 3 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/core/modules/comment/src/CommentViewBuilder.php b/core/modules/comment/src/CommentViewBuilder.php
index cf8b7b2..524349d 100644
--- a/core/modules/comment/src/CommentViewBuilder.php
+++ b/core/modules/comment/src/CommentViewBuilder.php
@@ -73,9 +73,12 @@ public function __construct(EntityTypeInterface $entity_type, EntityManagerInter
   protected function getBuildDefaults(EntityInterface $entity, $view_mode, $langcode) {
     $build = parent::getBuildDefaults($entity, $view_mode, $langcode);
 
-    // If threading is enabled, don't render cache individual comments.
-    if ($entity->getCommentedEntity()->getFieldDefinition($entity->getFieldName())->getSetting('default_mode')) {
-      unset($build['#cache']);
+    // If threading is enabled, don't render cache individual comments, but do
+    // keep the cache tags, so they can bubble up.
+    if ($entity->getCommentedEntity()->getFieldDefinition($entity->getFieldName())->getSetting('default_mode') === CommentManagerInterface::COMMENT_MODE_THREADED) {
+      $cache_tags = $build['#cache']['tags'];
+      $build['#cache'] = [];
+      $build['#cache']['tags'] = $cache_tags;
     }
 
     return $build;
diff --git a/core/modules/comment/src/Tests/CommentCacheTagsTest.php b/core/modules/comment/src/Tests/CommentCacheTagsTest.php
index c15bd29..ee69cc0 100644
--- a/core/modules/comment/src/Tests/CommentCacheTagsTest.php
+++ b/core/modules/comment/src/Tests/CommentCacheTagsTest.php
@@ -7,7 +7,9 @@
 
 namespace Drupal\comment\Tests;
 
+use Drupal\comment\CommentManagerInterface;
 use Drupal\Core\Entity\EntityInterface;
+use Drupal\field\Entity\FieldConfig;
 use Drupal\system\Tests\Entity\EntityWithUriCacheTagsTestBase;
 
 /**
@@ -44,7 +46,12 @@ protected function createEntity() {
     entity_test_create_bundle($bundle, NULL, 'entity_test');
 
     // Create a comment field on this bundle.
-    \Drupal::service('comment.manager')->addDefaultField('entity_test', 'bar');
+    \Drupal::service('comment.manager')->addDefaultField('entity_test', 'bar', 'comment');
+
+    // Display comments in a flat list; threaded comments are not render cached.
+    $field = FieldConfig::loadByName('entity_test', 'bar', 'comment');
+    $field->settings['default_mode'] = CommentManagerInterface::COMMENT_MODE_FLAT;
+    $field->save();
 
     // Create a "Camelids" test entity.
     $entity_test = entity_create('entity_test', array(
@@ -78,7 +85,6 @@ protected function createEntity() {
   protected function getAdditionalCacheTagsForEntity(EntityInterface $entity) {
     /** @var \Drupal\comment\CommentInterface $entity */
     return array(
-      'entity_test:1',
       'filter_format:plain_text',
       'user:' . $entity->getOwnerId(),
       'user_view:1',
diff --git a/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php b/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php
index af169ff..e5c1c41 100644
--- a/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php
+++ b/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php
@@ -102,10 +102,7 @@ public function testCacheTags() {
     drupal_render($build);
     $expected_cache_tags = array(
       'entity_test_view' => TRUE,
-      // The tags of the commented entity are duplicated in
-      // drupal_merge_cache_tags().
-      // @todo Remove the duplicate in https://www.drupal.org/node/2340123
-      'entity_test' => array($commented_entity->id(), $commented_entity->id()),
+      'entity_test' => array($commented_entity->id()),
       'comment_view' => TRUE,
       'comment' => array(1 => $comment->id()),
       'filter_format' => array(