diff --git a/core/.MAINTAINERS.txt.swp b/core/.MAINTAINERS.txt.swp
new file mode 100644
index 0000000..7043893
Binary files /dev/null and b/core/.MAINTAINERS.txt.swp differ
diff --git a/core/lib/Drupal/Core/Asset/AssetResolver.php b/core/lib/Drupal/Core/Asset/AssetResolver.php
index c41e14e..d0f00cf 100644
--- a/core/lib/Drupal/Core/Asset/AssetResolver.php
+++ b/core/lib/Drupal/Core/Asset/AssetResolver.php
@@ -123,23 +123,9 @@ public function getCssAssets(AttachedAssetsInterface $assets, $optimize) {
           // order.
           $options['weight'] += count($css) / 1000;
 
-          // Add the data to the CSS array depending on the type.
-          switch ($options['type']) {
-            case 'file':
-              // Local CSS files are keyed by basename; if a file with the same
-              // basename is added more than once, it gets overridden.
-              // By default, take over the filename as basename.
-              if (!isset($options['basename'])) {
-                $options['basename'] = drupal_basename($options['data']);
-              }
-              $css[$options['basename']] = $options;
-              break;
-
-            default:
-              // External files are keyed by their full URI, so the same CSS
-              // file is not added twice.
-              $css[$options['data']] = $options;
-          }
+          // Local and external files must keep their name as the associative
+          // key so the same CSS file is not added twice.
+          $css[$options['data']] = $options;
         }
       }
     }
@@ -155,15 +141,23 @@ public function getCssAssets(AttachedAssetsInterface $assets, $optimize) {
     if ($stylesheet_remove = $theme_info->getStyleSheetsRemove()) {
       foreach ($css as $key => $options) {
         if (isset($options['basename']) && isset($stylesheet_remove[$options['basename']])) {
-          unset($css[$key]);
+          foreach ($css as $css_path => $item) {
+            $file_name = end(explode('/', $css_path));
+            if ($file_name == $key) {
+              unset($css[$css_path]);  
+            }
+          }
         }
       }
     }
     // Allow themes to conditionally override CSS files by basename.
     if ($stylesheet_override = $theme_info->getStyleSheetsOverride()) {
       foreach ($css as $key => $options) {
-        if (isset($options['basename']) && isset($stylesheet_override[$options['basename']])) {
-          $css[$key]['data'] = $stylesheet_override[$options['basename']];
+        foreach ($css as $css_path => $item) {
+          $file_name = end(explode('/', $css_path));
+          if ($file_name == $key) {
+            $css[$css_path]['data'] = $stylesheet_override[$options['basename']];
+          }
         }
       }
     }
diff --git a/core/modules/system/src/Tests/Common/AttachedAssetsTest.php b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
index a52db9b..37b07b9 100644
--- a/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
+++ b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
@@ -89,7 +89,7 @@ function testAddFiles() {
 
     $css = $this->assetResolver->getCssAssets($assets, FALSE);
     $js = $this->assetResolver->getJsAssets($assets, FALSE)[1];
-    $this->assertTrue(array_key_exists('bar.css', $css), 'CSS files are correctly added.');
+    $this->assertTrue(array_key_exists('common_test/files/bar.css', $css), 'CSS files are correctly added.');
     $this->assertTrue(array_key_exists('core/modules/system/tests/modules/common_test/foo.js', $js), 'JavaScript files are correctly added.');
 
     $css_render_array = \Drupal::service('asset.css.collection_renderer')->render($css);
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: {}