diff --git a/core/modules/system/tests/themes/test_subsubtheme/test_subsubtheme.info.yml b/core/modules/system/tests/themes/test_subsubtheme/test_subsubtheme.info.yml
index 7bf373a..b803dd3 100644
--- a/core/modules/system/tests/themes/test_subsubtheme/test_subsubtheme.info.yml
+++ b/core/modules/system/tests/themes/test_subsubtheme/test_subsubtheme.info.yml
@@ -4,3 +4,12 @@ description: 'Test theme which uses test_subtheme as the base theme.'
 version: VERSION
 core: 8.x
 base theme: test_subtheme
+
+libraries-override:
+  core/drupal.dialog:
+    js:
+      misc/dialog/dialog.js: js/subsubtheme-dialog.js
+  core/jquery.farbtastic:
+    css:
+      component:
+        '@test_basetheme/css/farbtastic.css': css/subsubtheme-farbtastic.css
diff --git a/core/tests/Drupal/KernelTests/Core/Asset/LibraryDiscoveryIntegrationTest.php b/core/tests/Drupal/KernelTests/Core/Asset/LibraryDiscoveryIntegrationTest.php
index a77adfc..addf6d8 100644
--- a/core/tests/Drupal/KernelTests/Core/Asset/LibraryDiscoveryIntegrationTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Asset/LibraryDiscoveryIntegrationTest.php
@@ -161,6 +161,19 @@ public function testBaseThemeLibrariesOverrideInSubTheme() {
   }
 
   /**
+   * Tests library-override on already overridden libraries.
+   */
+  public function testLibrariesOverrideOverridden() {
+    // Activate a sub-theme that overrides a library override.
+    $this->activateTheme('test_subsubtheme');
+
+    // Assert that effective libraries override is from the test_subsubtheme.
+    $this->assertNoAssetInLibrary('core/modules/system/tests/themes/test_basetheme/css/farbtastic.css', 'core', 'jquery.farbtastic', 'css');
+    $this->assertAssetInLibrary('core/modules/system/tests/themes/test_subsubtheme/css/subsubtheme-farbtastic.css', 'core', 'jquery.farbtastic', 'css');
+    $this->assertAssetInLibrary('core/modules/system/tests/themes/test_subsubtheme/js/subsubtheme-dialog.js', 'core', 'drupal.dialog', 'js');
+  }
+
+  /**
    * Tests libraries-extend.
    */
   public function testLibrariesExtend() {