diff --git a/core/lib/Drupal/Core/Asset/AssetResolver.php b/core/lib/Drupal/Core/Asset/AssetResolver.php
index c41e14e..1301da9 100644
--- a/core/lib/Drupal/Core/Asset/AssetResolver.php
+++ b/core/lib/Drupal/Core/Asset/AssetResolver.php
@@ -126,13 +126,13 @@ public function getCssAssets(AttachedAssetsInterface $assets, $optimize) {
           // Add the data to the CSS array depending on the type.
           switch ($options['type']) {
             case 'file':
-              // Local CSS files are keyed by basename; if a file with the same
-              // basename is added more than once, it gets overridden.
-              // By default, take over the filename as basename.
+              // Local CSS files are keyed by library and the file basename.
+              // Same basename cannot be used multiple times in a single
+              // library.
               if (!isset($options['basename'])) {
                 $options['basename'] = drupal_basename($options['data']);
               }
-              $css[$options['basename']] = $options;
+              $css[$library . '/' . $options['basename']] = $options;
               break;
 
             default:
@@ -259,9 +259,23 @@ public function getJsAssets(AttachedAssetsInterface $assets, $optimize) {
           // order.
           $options['weight'] += count($javascript) / 1000;
 
-          // Local and external files must keep their name as the associative
-          // key so the same JavaScript file is not added twice.
-          $javascript[$options['data']] = $options;
+          // Add the data to the CSS array depending on the type.
+          switch ($options['type']) {
+            case 'file':
+              // Local Javascript files are keyed by library and basename.
+              // Same basename cannot be used multiple times in a single
+              // library.
+              if (!isset($options['basename'])) {
+                $options['basename'] = drupal_basename($options['data']);
+              }
+              $javascript[$library . '/' . $options['basename']] = $options;
+              break;
+
+            default:
+              // External files are keyed by their full URI, so the same
+              // Javascript file is not added twice.
+              $javascript[$options['data']] = $options;
+          }
         }
       }
     }
diff --git a/core/modules/system/src/Tests/Theme/ThemeInfoTest.php b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
index aeb8846..3b1361d 100644
--- a/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
@@ -99,13 +99,13 @@ public function testChanges() {
     $active_theme = $this->themeManager->getActiveTheme();
     // Make sure we are not testing the wrong theme.
     $this->assertEqual('test_theme', $active_theme->getName());
-    $this->assertEqual(['classy/base'], $active_theme->getLibraries());
+    $this->assertEqual(['classy/base', 'test_theme/global-styling'], $active_theme->getLibraries());
 
     // @see theme_test_system_info_alter()
     $this->state->set('theme_test.modify_info_files', TRUE);
     drupal_flush_all_caches();
     $active_theme = $this->themeManager->getActiveTheme();
-    $this->assertEqual(['classy/base', 'core/backbone'], $active_theme->getLibraries());
+    $this->assertEqual(['classy/base', 'test_theme/global-styling', 'core/backbone'], $active_theme->getLibraries());
   }
 
 }
diff --git a/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
index 4573d01..9aaa8e9 100644
--- a/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -181,6 +181,28 @@ function testCSSOverride() {
   }
 
   /**
+   * Ensures that test theme loads an CSS file named same as in parent theme. It
+   * should load both files.
+   *
+   * @see test_theme.info.yml
+   * @see test_theme.libraries.yml
+   */
+  function testCSSWithSameNameAsInParent() {
+    // Take off css preprocessing
+    $config = $this->config('system.performance');
+    $config->set('css.preprocess', 0);
+    $config->save();
+
+    // Reuse the same page as in testPreprocessForSuggestions(). Request page
+    // and ensure that parent and test theme's same named CSS file gets loaded.
+    $this->drupalGet('theme-test/suggestion');
+    $classy_layout_css_url = drupal_get_path('theme', 'classy') .'/css/layout.css';
+    $theme_test_layout_css_url = drupal_get_path('theme', 'test_theme') .'/css/layout.css';
+    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '" . $classy_layout_css_url . "')]")), $classy_layout_css_url . " found");
+    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '" . $theme_test_layout_css_url . "')]")), $theme_test_layout_css_url . " found");
+  }
+
+  /**
    * Ensures a themes template is overrideable based on the 'template' filename.
    */
   function testTemplateOverride() {
diff --git a/core/modules/system/tests/themes/test_theme/css/layout.css b/core/modules/system/tests/themes/test_theme/css/layout.css
new file mode 100644
index 0000000..db5765f
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/css/layout.css
@@ -0,0 +1,7 @@
+
+/**
+ * This file is for testing CSS file named same as in parent theme through
+ * *.libraries.yml file.
+ * Used in
+ * No contents are necessary.
+ */
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.info.yml b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
index 9eebc90..5000e74 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.info.yml
+++ b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
@@ -14,6 +14,8 @@ description: 'Theme for testing the theme system'
 version: VERSION
 base theme: classy
 core: 8.x
+libraries:
+  - test_theme/global-styling
 stylesheets-remove:
   - system.module.css
 regions:
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml b/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
new file mode 100644
index 0000000..ed21a2d
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
@@ -0,0 +1,5 @@
+global-styling:
+  version: VERSION
+  css:
+    theme:
+      css/layout.css: {}