diff --git a/core/lib/Drupal/Core/Asset/AssetResolver.php b/core/lib/Drupal/Core/Asset/AssetResolver.php
index c41e14e..1d43a79 100644
--- a/core/lib/Drupal/Core/Asset/AssetResolver.php
+++ b/core/lib/Drupal/Core/Asset/AssetResolver.php
@@ -123,23 +123,19 @@ public function getCssAssets(AttachedAssetsInterface $assets, $optimize) {
           // order.
           $options['weight'] += count($css) / 1000;
 
-          // Add the data to the CSS array depending on the type.
+          // Local and external files must keep their name as the associative
+          // key so the same CSS file is not added twice. Depending on the type,
+          // some options may need some processing.
           switch ($options['type']) {
             case 'file':
-              // Local CSS files are keyed by basename; if a file with the same
-              // basename is added more than once, it gets overridden.
-              // By default, take over the filename as basename.
+              // Local CSS files need the basename to be saved because it is
+              // used for overriding and removing stylesheets.
               if (!isset($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;
           }
+          $css[$options['data']] = $options;
         }
       }
     }
diff --git a/core/modules/system/src/Tests/Common/AttachedAssetsTest.php b/core/modules/system/src/Tests/Common/AttachedAssetsTest.php
index a52db9b..d111f44 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('core/modules/system/tests/modules/common_test/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);
@@ -470,7 +470,7 @@ function testAddJsFileWithQueryString() {
 
     $css = $this->assetResolver->getCssAssets($assets, FALSE);
     $js = $this->assetResolver->getJsAssets($assets, FALSE)[1];
-    $this->assertTrue(array_key_exists('querystring.css?arg1=value1&arg2=value2', $css), 'CSS file with query string is correctly added.');
+    $this->assertTrue(array_key_exists('core/modules/system/tests/modules/common_test/querystring.css?arg1=value1&arg2=value2', $css), 'CSS file with query string is correctly added.');
     $this->assertTrue(array_key_exists('core/modules/system/tests/modules/common_test/querystring.js?arg1=value1&arg2=value2', $js), 'JavaScript file with query string is 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..aa069c5 100644
--- a/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
@@ -86,6 +86,10 @@ function testStylesheets() {
     $this->assertIdentical(0, count($this->xpath("//link[contains(@href, 'sub-remove.css')]")), "sub-remove.css not found");
     $this->assertIdentical(0, count($this->xpath("//link[contains(@href, 'base-add.sub-remove.css')]")), "base-add.sub-remove.css not found");
     $this->assertIdentical(0, count($this->xpath("//link[contains(@href, 'base-override.sub-remove.css')]")), "base-override.sub-remove.css not found");
+
+    // Verify that CSS files with the same name are loaded from both the base theme and subtheme.
+    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '$base/samename.css')]")), "$base/samename.css found");
+    $this->assertIdentical(1, count($this->xpath("//link[contains(@href, '$sub/samename.css')]")), "$sub/samename.css found");
   }
 
   /**
@@ -99,13 +103,13 @@ 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/tests/themes/test_basetheme/test_basetheme.libraries.yml b/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml
index 1c72d30..49e4ba1 100644
--- a/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml
+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.libraries.yml
@@ -5,3 +5,4 @@ global-styling:
       base-add.css: {}
       base-add.sub-override.css: {}
       base-add.sub-remove.css: {}
+      samename.css: {}
diff --git a/core/modules/system/tests/themes/test_subtheme/test_subtheme.libraries.yml b/core/modules/system/tests/themes/test_subtheme/test_subtheme.libraries.yml
index efce7ae..931dffe 100644
--- a/core/modules/system/tests/themes/test_subtheme/test_subtheme.libraries.yml
+++ b/core/modules/system/tests/themes/test_subtheme/test_subtheme.libraries.yml
@@ -3,3 +3,4 @@ global-styling:
   css:
     base:
       css/sub-add.css: {}
+      css/samename.css: {}