diff --git a/lib/src/async_environment.dart b/lib/src/async_environment.dart
index 7bc7e824..c7e48939 100644
--- a/lib/src/async_environment.dart
+++ b/lib/src/async_environment.dart
@@ -632,6 +632,7 @@ class _EnvironmentModule implements AsyncModule {
   final Extender extender;
   final CssStylesheet css;
   final bool transitivelyContainsCss;
+  final bool transitivelyContainsExtensions;
 
   /// The environment that defines this module's members.
   final AsyncEnvironment _environment;
@@ -648,7 +649,10 @@ class _EnvironmentModule implements AsyncModule {
         mixins = PublicMemberMap(_environment._mixins.first),
         transitivelyContainsCss = css.children.isNotEmpty ||
             _environment._allModules
-                .any((module) => module.transitivelyContainsCss);
+                .any((module) => module.transitivelyContainsCss),
+        transitivelyContainsExtensions = !extender.isEmpty ||
+            _environment._allModules
+                .any((module) => module.transitivelyContainsExtensions);
 
   void setVariable(String name, Value value, AstNode nodeWithSpan) {
     if (!_environment._variables.first.containsKey(name)) {
diff --git a/lib/src/async_module.dart b/lib/src/async_module.dart
index 70b9a26e..7e0b4f70 100644
--- a/lib/src/async_module.dart
+++ b/lib/src/async_module.dart
@@ -58,6 +58,10 @@ abstract class AsyncModule {
   /// Whether this module *or* any modules in [upstream] contain any CSS.
   bool get transitivelyContainsCss;
 
+  /// Whether this module *or* any modules in [upstream] contain `@extend`
+  /// rules..
+  bool get transitivelyContainsExtensions;
+
   /// Sets the variable named [name] to [value], associated with
   /// [nodeWithSpan]'s source span.
   ///
diff --git a/lib/src/environment.dart b/lib/src/environment.dart
index 7d4eb35d..a57dee88 100644
--- a/lib/src/environment.dart
+++ b/lib/src/environment.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_environment.dart.
 // See tool/synchronize.dart for details.
 //
-// Checksum: 097eb94cd15103bf4bef739a61e61414db4b55b1
+// Checksum: 3210a5c0528eac456ae8ca7827b65f3976f6b29d
 //
 // ignore_for_file: unused_import
 
@@ -634,6 +634,7 @@ class _EnvironmentModule implements Module {
   final Extender extender;
   final CssStylesheet css;
   final bool transitivelyContainsCss;
+  final bool transitivelyContainsExtensions;
 
   /// The environment that defines this module's members.
   final Environment _environment;
@@ -650,7 +651,10 @@ class _EnvironmentModule implements Module {
         mixins = PublicMemberMap(_environment._mixins.first),
         transitivelyContainsCss = css.children.isNotEmpty ||
             _environment._allModules
-                .any((module) => module.transitivelyContainsCss);
+                .any((module) => module.transitivelyContainsCss),
+        transitivelyContainsExtensions = !extender.isEmpty ||
+            _environment._allModules
+                .any((module) => module.transitivelyContainsExtensions);
 
   void setVariable(String name, Value value, AstNode nodeWithSpan) {
     if (!_environment._variables.first.containsKey(name)) {
diff --git a/lib/src/module.dart b/lib/src/module.dart
index 5fb0fc53..67167596 100644
--- a/lib/src/module.dart
+++ b/lib/src/module.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_module.dart.
 // See tool/synchronize.dart for details.
 //
-// Checksum: 895440529b78f1ef2830cf105af00dba0755e947
+// Checksum: d996840504b080c2c4e6b34136562b8152bb2e97
 //
 // ignore_for_file: unused_import
 
@@ -65,6 +65,10 @@ abstract class Module {
   /// Whether this module *or* any modules in [upstream] contain any CSS.
   bool get transitivelyContainsCss;
 
+  /// Whether this module *or* any modules in [upstream] contain `@extend`
+  /// rules..
+  bool get transitivelyContainsExtensions;
+
   /// Sets the variable named [name] to [value], associated with
   /// [nodeWithSpan]'s source span.
   ///
diff --git a/lib/src/visitor/async_evaluate.dart b/lib/src/visitor/async_evaluate.dart
index 0e9179c3..8fbc8f1f 100644
--- a/lib/src/visitor/async_evaluate.dart
+++ b/lib/src/visitor/async_evaluate.dart
@@ -1033,7 +1033,12 @@ class _EvaluateVisitor
     var module = environment.toModule(
         CssStylesheet(const [], stylesheet.span), Extender.empty);
     if (module.transitivelyContainsCss) {
-      await _combineCss(module, clone: true).accept(this);
+      // If any transitively used module contains extensions, we need to clone
+      // all modules' CSS. Otherwise, it's possible that they'll be used or
+      // imported from another location that shouldn't have the same extensions
+      // applied.
+      await _combineCss(module, clone: module.transitivelyContainsExtensions)
+          .accept(this);
     }
 
     var visitor = _ImportedCssVisitor(this);
diff --git a/lib/src/visitor/evaluate.dart b/lib/src/visitor/evaluate.dart
index 92a0e919..44153ce9 100644
--- a/lib/src/visitor/evaluate.dart
+++ b/lib/src/visitor/evaluate.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/synchronize.dart for details.
 //
-// Checksum: 0f4f00db38a7f2a60771d3d50652cbfdeb322e52
+// Checksum: 4de442e73c75611a675e2fd0d8962219f1ceffd7
 //
 // ignore_for_file: unused_import
 
@@ -1036,7 +1036,12 @@ class _EvaluateVisitor
     var module = environment.toModule(
         CssStylesheet(const [], stylesheet.span), Extender.empty);
     if (module.transitivelyContainsCss) {
-      _combineCss(module, clone: true).accept(this);
+      // If any transitively used module contains extensions, we need to clone
+      // all modules' CSS. Otherwise, it's possible that they'll be used or
+      // imported from another location that shouldn't have the same extensions
+      // applied.
+      _combineCss(module, clone: module.transitivelyContainsExtensions)
+          .accept(this);
     }
 
     var visitor = _ImportedCssVisitor(this);