diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index 4bbffc3..3c12cdd 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -209,6 +209,32 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
       return '';
     }
 
+    // Render only the children if the internal #render_children property is
+    // set.
+    // @see \Drupal\Core\Theme\ThemeManager::render().
+    if (isset($elements['#render_children'])) {
+      // A non-empty #children property takes precedence. This happens only if
+      // it has been manually set into the render array.
+      if (!empty($elements['#children'])) {
+        $children = ['#children'];
+      }
+      else {
+        $children = Element::children($elements);
+      }
+
+      if (empty($children)) {
+        return '';
+      }
+
+      // Avoid making this change for the referenced elements variable because
+      // it could cause unexpected behaviour with other templating engines than
+      // Twig. Save the original referenced variable to allow making changes for
+      // the referenced variable in case explicitly wanted so.
+      $original_elements = &$elements;
+      $new_elements = array_intersect_key($elements, array_flip($children));
+      $elements = &$new_elements;
+    }
+
     if (!isset($elements['#access']) && isset($elements['#access_callback'])) {
       if (is_string($elements['#access_callback']) && strpos($elements['#access_callback'], '::') === FALSE) {
         $elements['#access_callback'] = $this->controllerResolver->getControllerFromDefinition($elements['#access_callback']);
@@ -428,10 +454,8 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
     }
 
     // Call the element's #theme function if it is set. Then any children of the
-    // element have to be rendered there. If the internal #render_children
-    // property is set, do not call the #theme function to prevent infinite
-    // recursion.
-    if ($theme_is_implemented && !isset($elements['#render_children'])) {
+    // element have to be rendered there.
+    if ($theme_is_implemented) {
       $elements['#children'] = $this->theme->render($elements['#theme'], $elements);
 
       // If ThemeManagerInterface::render() returns FALSE this means that the
@@ -440,10 +464,10 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
       $theme_is_implemented = ($elements['#children'] !== FALSE);
     }
 
-    // If #theme is not implemented or #render_children is set and the element
-    // has an empty #children attribute, render the children now. This is the
-    // same process as Renderer::render() but is inlined for speed.
-    if ((!$theme_is_implemented || isset($elements['#render_children'])) && empty($elements['#children'])) {
+    // If #theme is not implemented and the element has an empty #children
+    // attribute, render the children now. This is the same process as
+    // Renderer::render() but is inlined for speed.
+    if (!$theme_is_implemented && empty($elements['#children'])) {
       foreach ($children as $key) {
         $elements['#children'] .= $this->doRender($elements[$key]);
       }
@@ -467,9 +491,7 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
     // because the #type 'page' render array from drupal_prepare_page() would
     // render the $page and wrap it into the html.html.twig template without the
     // attached assets otherwise.
-    // If the internal #render_children property is set, do not call the
-    // #theme_wrappers function(s) to prevent infinite recursion.
-    if (isset($elements['#theme_wrappers']) && !isset($elements['#render_children'])) {
+    if (isset($elements['#theme_wrappers'])) {
       foreach ($elements['#theme_wrappers'] as $key => $value) {
         // If the value of a #theme_wrappers item is an array then the theme
         // hook is found in the key of the item and the value contains attribute
@@ -513,6 +535,13 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
 
     $elements['#markup'] = Markup::create($prefix . $elements['#children'] . $suffix);
 
+    // #attached and #markup values should be always saved to the referenced
+    // elements variable.
+    if (isset($original_elements)) {
+      $original_elements['#markup'] = $elements['#markup'];
+      $original_elements['#attached'] = $elements['#attached'];
+    }
+
     // We've rendered this element (and its subtree!), now update the context.
     $context->update($elements);
 
diff --git a/core/modules/file/src/Tests/FileFieldValidateTest.php b/core/modules/file/src/Tests/FileFieldValidateTest.php
index 9d43060..de3b1c5 100644
--- a/core/modules/file/src/Tests/FileFieldValidateTest.php
+++ b/core/modules/file/src/Tests/FileFieldValidateTest.php
@@ -71,8 +71,10 @@ function testFileMaxSize() {
     $field_name = strtolower($this->randomMachineName());
     $this->createFileField($field_name, 'node', $type_name, array(), array('required' => '1'));
 
-    $small_file = $this->getTestFile('text', 131072); // 128KB.
-    $large_file = $this->getTestFile('text', 1310720); // 1.2MB
+    // 128KB.
+    $small_file = $this->getTestFile('text', 131072);
+    // 1.2MB.
+    $large_file = $this->getTestFile('text', 1310720);
 
     // Test uploading both a large and small file with different increments.
     $sizes = array(
@@ -185,4 +187,25 @@ public function testFileRemoval() {
     $this->assertText('Article ' . $node->getTitle() . ' has been updated.');
   }
 
+  /**
+   * Test the validation message is displayed only once for ajax uploads.
+   */
+  public function testAJAXValidationMessage() {
+    $field_name = strtolower($this->randomMachineName());
+    $this->createFileField($field_name, 'node', 'article');
+
+    $this->drupalGet('node/add/article');
+    /** @var \Drupal\file\FileInterface $image_file */
+    $image_file = $this->getTestFile('image');
+    $edit = array(
+      'files[' . $field_name . '_0]' => $this->container->get('file_system')->realpath($image_file->getFileUri()),
+      'title[0][value]' => $this->randomMachineName(),
+    );
+    $this->drupalPostAjaxForm(NULL, $edit, $field_name . '_0_upload_button');
+    $elements = $this->xpath('//div[contains(@class, :class)]', array(
+      ':class' => 'messages--error',
+    ));
+    $this->assertEqual(count($elements), 1, 'Ajax validation messages are displayed once.');
+  }
+
 }
diff --git a/core/modules/image/src/Tests/ImageFieldValidateTest.php b/core/modules/image/src/Tests/ImageFieldValidateTest.php
index 5f0bf21..bf6b4df 100644
--- a/core/modules/image/src/Tests/ImageFieldValidateTest.php
+++ b/core/modules/image/src/Tests/ImageFieldValidateTest.php
@@ -8,6 +8,7 @@
  * @group image
  */
 class ImageFieldValidateTest extends ImageFieldTestBase {
+
   /**
    * Test min/max resolution settings.
    */
@@ -156,4 +157,26 @@ protected function getFieldSettings($min_resolution, $max_resolution) {
     ];
   }
 
+  /**
+   * Test the validation message is displayed only once for ajax uploads.
+   */
+  public function testAJAXValidationMessage() {
+    $field_name = strtolower($this->randomMachineName());
+    $this->createImageField($field_name, 'article');
+
+    $this->drupalGet('node/add/article');
+    /** @var \Drupal\file\FileInterface[] $text_files */
+    $text_files = $this->drupalGetTestFiles('text');
+    $text_file = reset($text_files);
+    $edit = array(
+      'files[' . $field_name . '_0]' => $this->container->get('file_system')->realpath($text_file->uri),
+      'title[0][value]' => $this->randomMachineName(),
+    );
+    $this->drupalPostAjaxForm(NULL, $edit, $field_name . '_0_upload_button');
+    $elements = $this->xpath('//div[contains(@class, :class)]', array(
+      ':class' => 'messages--error',
+    ));
+    $this->assertEqual(count($elements), 1, 'Ajax validation messages are displayed once.');
+  }
+
 }
diff --git a/core/tests/Drupal/KernelTests/Core/Render/RenderTest.php b/core/tests/Drupal/KernelTests/Core/Render/RenderTest.php
index 8860c19..6bb3bed 100644
--- a/core/tests/Drupal/KernelTests/Core/Render/RenderTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Render/RenderTest.php
@@ -16,7 +16,7 @@ class RenderTest extends KernelTestBase {
    *
    * @var array
    */
-  public static $modules = array('system', 'common_test');
+  public static $modules = array('system', 'common_test', 'theme_test');
 
   /**
    * Tests theme preprocess functions being able to attach assets.
@@ -44,6 +44,23 @@ function testDrupalRenderThemePreprocessAttached() {
   }
 
   /**
+   * Ensures that render array children are processed correctly.
+   */
+  public function testRenderChildren() {
+    // Ensure that #prefix and #suffix is only being printed once since that is
+    // the behaviour the caller code expects.
+    $build = [
+      '#type' => 'container',
+      '#theme' => 'theme_test_render_element_children',
+      '#prefix' => 'kangaroo',
+      '#suffix' => 'kitten',
+    ];
+    $this->render($build);
+    $this->removeWhiteSpace();
+    $this->assertNoRaw('<div>kangarookitten</div>');
+  }
+
+  /**
    * Tests that we get an exception when we try to attach an illegal type.
    */
   public function testProcessAttached() {
diff --git a/core/tests/Drupal/Tests/Core/Render/RendererTest.php b/core/tests/Drupal/Tests/Core/Render/RendererTest.php
index 51cfde8..7e24134 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererTest.php
@@ -363,6 +363,25 @@ public function providerTestRenderBasic() {
     };
     $data[] = [$build, 'baz', $setup_code];
 
+    // #theme is implemented but #render_children is TRUE. In this case the
+    // calling code is expecting only the children to be rendered. #prefix and
+    // #suffix should not be inherited for the children.
+    $build = [
+      '#theme' => 'common_test_foo',
+      '#children' => '',
+      '#prefix' => 'kangaroo',
+      '#suffix' => 'unicorn',
+      '#render_children' => TRUE,
+      'child' => [
+        '#markup' => 'kitten',
+      ],
+    ];
+    $setup_code = function() {
+      $this->themeManager->expects($this->never())
+        ->method('render');
+    };
+    $data[] = [$build, 'kitten', $setup_code];
+
     return $data;
   }