core/includes/common.inc                                         | 4 ++--
 core/lib/Drupal/Core/Cache/ApcuBackend.php                       | 2 +-
 core/lib/Drupal/Core/Cache/Cache.php                             | 2 +-
 core/lib/Drupal/Core/Cache/DatabaseBackend.php                   | 4 ++--
 core/lib/Drupal/Core/Cache/MemoryBackend.php                     | 3 +--
 core/lib/Drupal/Core/Cache/PhpBackend.php                        | 3 +--
 core/lib/Drupal/Core/Entity/Entity.php                           | 3 +--
 core/lib/Drupal/Core/Entity/EntityViewBuilder.php                | 1 -
 core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php        | 7 ++++++-
 core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php | 5 ++---
 core/modules/block/src/BlockViewBuilder.php                      | 1 -
 core/modules/block/src/Tests/BlockViewBuilderTest.php            | 1 -
 core/modules/block/tests/modules/block_test/block_test.module    | 2 +-
 core/modules/system/src/Plugin/Block/SystemBrandingBlock.php     | 6 +++---
 core/modules/system/src/Plugin/Block/SystemMenuBlock.php         | 5 +++--
 core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php | 4 +---
 16 files changed, 25 insertions(+), 28 deletions(-)

diff --git a/core/includes/common.inc b/core/includes/common.inc
index 24ac99a..9bd3038 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -29,7 +29,6 @@
 use Drupal\Core\PhpStorage\PhpStorageFactory;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Datetime\DrupalDateTime;
-use Drupal\Core\EventSubscriber\HtmlViewSubscriber;
 use Drupal\Core\Routing\GeneratorNotInitializedException;
 use Drupal\Core\Template\Attribute;
 use Drupal\Core\Render\Element;
@@ -3118,7 +3117,8 @@ function drupal_render_cache_set(&$markup, array $elements) {
   // Persist cache tags associated with this element. Also associate the
   // "rendered" cache tag. This allows us to invalidate the entire render cache,
   // regardless of the cache bin.
-  $data['#cache']['tags'] = Cache::mergeTags($elements['#cache']['tags'], ['rendered']);
+  $data['#cache']['tags'] = $elements['#cache']['tags'];
+  $data['#cache']['tags'][] = 'rendered';
 
   $bin = isset($elements['#cache']['bin']) ? $elements['#cache']['bin'] : 'render';
   $expire = isset($elements['#cache']['expire']) ? $elements['#cache']['expire'] : Cache::PERMANENT;
diff --git a/core/lib/Drupal/Core/Cache/ApcuBackend.php b/core/lib/Drupal/Core/Cache/ApcuBackend.php
index f787401..199a134 100644
--- a/core/lib/Drupal/Core/Cache/ApcuBackend.php
+++ b/core/lib/Drupal/Core/Cache/ApcuBackend.php
@@ -189,7 +189,7 @@ protected function prepareItem($cache, $allow_invalid) {
    * {@inheritdoc}
    */
   public function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array()) {
-    Cache::validateTags($tags);
+    $tags = Cache::mergeTags($tags, []);
     $cache = new \stdClass();
     $cache->cid = $cid;
     $cache->created = round(microtime(TRUE), 3);
diff --git a/core/lib/Drupal/Core/Cache/Cache.php b/core/lib/Drupal/Core/Cache/Cache.php
index 68d316c..a29ae57 100644
--- a/core/lib/Drupal/Core/Cache/Cache.php
+++ b/core/lib/Drupal/Core/Cache/Cache.php
@@ -22,7 +22,7 @@ class Cache {
   const PERMANENT = CacheBackendInterface::CACHE_PERMANENT;
 
   /**
-   * Merges arrays of cache tags.
+   * Merges arrays of cache tags and removes duplicates.
    *
    * The cache tags array is returned in a format that is valid for
    * \Drupal\Core\Cache\CacheBackendInterface::set().
diff --git a/core/lib/Drupal/Core/Cache/DatabaseBackend.php b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
index b243967..480c840 100644
--- a/core/lib/Drupal/Core/Cache/DatabaseBackend.php
+++ b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
@@ -147,7 +147,7 @@ protected function prepareItem($cache, $allow_invalid) {
    * Implements Drupal\Core\Cache\CacheBackendInterface::set().
    */
   public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array()) {
-    Cache::validateTags($tags);
+    $tags = Cache::mergeTags($tags, []);
     $try_again = FALSE;
     try {
       // The bin might not yet exist.
@@ -234,7 +234,7 @@ public function setMultiple(array $items) {
           'tags' => array(),
         );
 
-        Cache::validateTags($item['tags']);
+        $item['tags'] = Cache::mergeTags($item['tags'], []);
 
         // Remove tags that were already deleted or invalidated during this
         // request from the static caches so that another deletion or
diff --git a/core/lib/Drupal/Core/Cache/MemoryBackend.php b/core/lib/Drupal/Core/Cache/MemoryBackend.php
index a8814ac..ce74ff4 100644
--- a/core/lib/Drupal/Core/Cache/MemoryBackend.php
+++ b/core/lib/Drupal/Core/Cache/MemoryBackend.php
@@ -107,13 +107,12 @@ protected function prepareItem($cache, $allow_invalid) {
    * Implements Drupal\Core\Cache\CacheBackendInterface::set().
    */
   public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array()) {
-    Cache::validateTags($tags);
     $this->cache[$cid] = (object) array(
       'cid' => $cid,
       'data' => serialize($data),
       'created' => REQUEST_TIME,
       'expire' => $expire,
-      'tags' => $tags,
+      'tags' => Cache::mergeTags($tags, []),
     );
   }
 
diff --git a/core/lib/Drupal/Core/Cache/PhpBackend.php b/core/lib/Drupal/Core/Cache/PhpBackend.php
index 26eaf7e..e7de3b6 100644
--- a/core/lib/Drupal/Core/Cache/PhpBackend.php
+++ b/core/lib/Drupal/Core/Cache/PhpBackend.php
@@ -138,9 +138,8 @@ public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array
       'data' => $data,
       'created' => round(microtime(TRUE), 3),
       'expire' => $expire,
-      'tags' => $tags,
+      'tags' => Cache::mergeTags($tags, []),
     );
-    Cache::validateTags($item->tags);
     $this->writeItem($this->normalizeCid($cid), $item);
   }
 
diff --git a/core/lib/Drupal/Core/Entity/Entity.php b/core/lib/Drupal/Core/Entity/Entity.php
index a2cf4fa..fc289d2 100644
--- a/core/lib/Drupal/Core/Entity/Entity.php
+++ b/core/lib/Drupal/Core/Entity/Entity.php
@@ -9,7 +9,6 @@
 
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\DependencyInjection\DependencySerializationTrait;
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Config\Entity\Exception\ConfigEntityIdLengthException;
@@ -473,7 +472,7 @@ protected function invalidateTagsOnSave($update) {
     $tags = $this->getListCacheTags();
     if ($update) {
       // An existing entity was updated, also invalidate its unique cache tag.
-      $tags = NestedArray::mergeDeep($tags, $this->getCacheTag());
+      $tags = Cache::mergeTags($tags, $this->getCacheTag());
       $this->onUpdateBundleEntity();
     }
     Cache::invalidateTags($tags);
diff --git a/core/lib/Drupal/Core/Entity/EntityViewBuilder.php b/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
index e989182..e24735a 100644
--- a/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
+++ b/core/lib/Drupal/Core/Entity/EntityViewBuilder.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\Core\Entity;
 
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Entity\Display\EntityViewDisplayInterface;
 use Drupal\Core\Field\FieldItemInterface;
diff --git a/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php b/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
index 77bfc3c..164f7f5 100644
--- a/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
+++ b/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
@@ -72,7 +72,12 @@ public function render(HtmlFragmentInterface $fragment, $status_code = 200) {
       // Persist cache tags associated with this page. Also associate the
       // "rendered" cache tag. This allows us to invalidate the entire render
       // cache, regardless of the cache bin.
-      $page->setCacheTags(Cache::mergeTags($page_array['#cache']['tags'], ['rendered']));
+      $cache_tags = $page_array['#cache']['tags'];
+      $cache_tags[] = 'rendered';
+      // Only keep unique cache tags. We need to prevent duplicates here already
+      // rather than only in the cache layer, because they are also used by
+      // reverse proxies (like Varnish), not only by Drupal's page cache.
+      $page->setCacheTags(array_unique($cache_tags));
     }
 
     return $page;
diff --git a/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php b/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php
index 65f95f1..f28ff0d 100644
--- a/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php
+++ b/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php
@@ -7,10 +7,10 @@
 
 namespace Drupal\aggregator\Plugin\Block;
 
-use Drupal\Component\Utility\NestedArray;
 use Drupal\aggregator\FeedStorageInterface;
 use Drupal\aggregator\ItemStorageInterface;
 use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Cache\Cache;
 use Drupal\Core\Entity\Query\QueryInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
@@ -195,8 +195,7 @@ public function build() {
   public function getCacheTags() {
     $cache_tags = parent::getCacheTags();
     $feed = $this->feedStorage->load($this->configuration['feed']);
-    $cache_tags = NestedArray::mergeDeep($cache_tags, $feed->getCacheTag());
-    return $cache_tags;
+    return Cache::mergeTags($cache_tags, $feed->getCacheTag());
   }
 
 }
diff --git a/core/modules/block/src/BlockViewBuilder.php b/core/modules/block/src/BlockViewBuilder.php
index c3d6147..25a2a2c 100644
--- a/core/modules/block/src/BlockViewBuilder.php
+++ b/core/modules/block/src/BlockViewBuilder.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\block;
 
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Component\Utility\String;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Entity\EntityViewBuilder;
diff --git a/core/modules/block/src/Tests/BlockViewBuilderTest.php b/core/modules/block/src/Tests/BlockViewBuilderTest.php
index a115c6d..b2dde86 100644
--- a/core/modules/block/src/Tests/BlockViewBuilderTest.php
+++ b/core/modules/block/src/Tests/BlockViewBuilderTest.php
@@ -8,7 +8,6 @@
 namespace Drupal\block\Tests;
 
 use Drupal\Component\Utility\Html;
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\UrlCacheContext;
 use Drupal\simpletest\DrupalUnitTestBase;
diff --git a/core/modules/block/tests/modules/block_test/block_test.module b/core/modules/block/tests/modules/block_test/block_test.module
index 19e65ad..cec0b24 100644
--- a/core/modules/block/tests/modules/block_test/block_test.module
+++ b/core/modules/block/tests/modules/block_test/block_test.module
@@ -28,7 +28,7 @@ function block_test_block_view_test_cache_alter(array &$build, BlockPluginInterf
     $build['#cache']['keys'][] = \Drupal::state()->get('block_test_view_alter_cache_key');
   }
   if (\Drupal::state()->get('block_test_view_alter_cache_tag') !== NULL) {
-    $build['#cache']['tags'] = Cache::mergeTags($build['#cache']['tags'], [\Drupal::state()->get('block_test_view_alter_cache_tag')]);
+    $build['#cache']['tags'][] = \Drupal::state()->get('block_test_view_alter_cache_tag');
   }
   if (\Drupal::state()->get('block_test_view_alter_append_pre_render_prefix') !== NULL) {
     $build['#pre_render'][] = 'block_test_pre_render_alter_content';
diff --git a/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php b/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
index e65109e..6d10aff 100644
--- a/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
@@ -7,9 +7,7 @@
 
 namespace Drupal\system\Plugin\Block;
 
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Block\BlockBase;
-use Drupal\Core\Cache\Cache;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
@@ -193,7 +191,9 @@ public function build() {
   public function getCacheTags() {
     // The theme-specific cache tag is set automatically for each block, but the
     // output of this block also depends on the global theme settings.
-    return Cache::mergeTags(parent::getCacheTags(), ['theme_global_setting']);
+    $cache_tags = parent::getCacheTags();
+    $cache_tags[] = 'theme_global_setting';
+    return $cache_tags;
   }
 
   /**
diff --git a/core/modules/system/src/Plugin/Block/SystemMenuBlock.php b/core/modules/system/src/Plugin/Block/SystemMenuBlock.php
index fc57930..16af329 100644
--- a/core/modules/system/src/Plugin/Block/SystemMenuBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemMenuBlock.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\system\Plugin\Block;
 
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Block\BlockBase;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Form\FormStateInterface;
@@ -190,7 +189,9 @@ public function getCacheTags() {
     // the cache tag for this menu to be set: whenever the menu is changed, this
     // menu block must also be re-rendered for that user, because maybe a menu
     // link that is accessible for that user has been added.
-    return Cache::mergeTags(parent::getCacheTags(), ['menu:' . $this->getDerivativeId()]);
+    $cache_tags = parent::getCacheTags();
+    $cache_tags[] = 'menu:' . $this->getDerivativeId();
+    return $cache_tags;
   }
 
   /**
diff --git a/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
index c2db659..1fa0d93 100644
--- a/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
+++ b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
@@ -7,10 +7,8 @@
 
 namespace Drupal\system\Tests\Entity;
 
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Entity\EntityInterface;
-use Drupal\Core\EventSubscriber\HtmlViewSubscriber;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\system\Tests\Cache\PageCacheTagsTestBase;
 
@@ -467,7 +465,7 @@ public function testReferencedEntity() {
     $this->verifyPageCache($non_referencing_entity_path, 'HIT');
 
     // Verify cache hits.
-    $referencing_entity_cache_tags = NestedArray::mergeDeep(
+    $referencing_entity_cache_tags = Cache::mergeTags(
       $this->referencing_entity->getCacheTag(),
       \Drupal::entityManager()->getViewBuilder('entity_test')->getCacheTag()
     );