diff --git a/Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.cpp b/Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.cpp
index 338c0919e2..9422329129 100644
--- a/Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.cpp
@@ -18,6 +18,20 @@ namespace JS::Intl {
 // 6.2.2 IsStructurallyValidLanguageTag ( locale ), https://tc39.es/ecma402/#sec-isstructurallyvalidlanguagetag
 static Optional<Unicode::LocaleID> is_structurally_valid_language_tag(StringView locale)
 {
+    auto contains_duplicate_variant = [](Vector<StringView>& variants) {
+        if (variants.is_empty())
+            return false;
+
+        quick_sort(variants);
+
+        for (size_t i = 0; i < variants.size() - 1; ++i) {
+            if (variants[i] == variants[i + 1])
+                return true;
+        }
+
+        return false;
+    };
+
     // IsStructurallyValidLanguageTag returns true if all of the following conditions hold, false otherwise:
 
     // locale can be generated from the EBNF grammar for unicode_locale_id in Unicode Technical Standard #35 LDML § 3.2 Unicode Locale Identifier;
@@ -31,23 +45,33 @@ static Optional<Unicode::LocaleID> is_structurally_valid_language_tag(StringView
         return {};
 
     // the unicode_language_id within locale contains no duplicate unicode_variant_subtag subtags; and
-    if (auto& variants = locale_id->language_id.variants; !variants.is_empty()) {
-        quick_sort(variants);
+    if (contains_duplicate_variant(locale_id->language_id.variants))
+        return {};
 
-        for (size_t i = 0; i < variants.size() - 1; ++i) {
-            if (variants[i] == variants[i + 1])
+    // if locale contains an extensions* component, that component
+    Vector<char> unique_keys;
+    for (auto& extension : locale_id->extensions) {
+        // does not contain any other_extensions components with duplicate [alphanum-[tTuUxX]] subtags,
+        // contains at most one unicode_locale_extensions component,
+        // contains at most one transformed_extensions component, and
+        char key = extension.visit(
+            [](Unicode::LocaleExtension const&) { return 'u'; },
+            [](Unicode::TransformedExtension const&) { return 't'; },
+            [](Unicode::OtherExtension const& ext) { return static_cast<char>(to_ascii_lowercase(ext.key)); });
+
+        if (unique_keys.contains_slow(key))
+            return {};
+        unique_keys.append(key);
+
+        // if a transformed_extensions component that contains a tlang component is present, then
+        // the tlang component contains no duplicate unicode_variant_subtag subtags.
+        if (auto* transformed = extension.get_pointer<Unicode::TransformedExtension>()) {
+            auto& language = transformed->language;
+            if (language.has_value() && contains_duplicate_variant(language->variants))
                 return {};
         }
     }
 
-    // FIXME: Handle extensions.
-    // if locale contains an extensions* component, that component
-    //     does not contain any other_extensions components with duplicate [alphanum-[tTuUxX]] subtags,
-    //     contains at most one unicode_locale_extensions component,
-    //     contains at most one transformed_extensions component, and
-    //     if a transformed_extensions component that contains a tlang component is present, then
-    //         the tlang component contains no duplicate unicode_variant_subtag subtags.
-
     return locale_id;
 }
 
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Intl/Intl.getCanonicalLocales.js b/Userland/Libraries/LibJS/Tests/builtins/Intl/Intl.getCanonicalLocales.js
index c7e64c1468..c248dda559 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Intl/Intl.getCanonicalLocales.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Intl/Intl.getCanonicalLocales.js
@@ -45,6 +45,29 @@ describe("errors", () => {
             Intl.getCanonicalLocales([true]);
         }).toThrowWithMessage(TypeError, "true is neither an object nor a string");
     });
+
+    test("duplicate extension components", () => {
+        expect(() => {
+            Intl.getCanonicalLocales("en-u-aa-U-aa");
+        }).toThrowWithMessage(RangeError, "en-u-aa-U-aa is not a structurally valid language tag");
+
+        expect(() => {
+            Intl.getCanonicalLocales("en-t-aa-T-aa");
+        }).toThrowWithMessage(RangeError, "en-t-aa-T-aa is not a structurally valid language tag");
+
+        expect(() => {
+            Intl.getCanonicalLocales("en-z-aa-Z-aa");
+        }).toThrowWithMessage(RangeError, "en-z-aa-Z-aa is not a structurally valid language tag");
+    });
+
+    test("duplicate transformed extension variant subtags", () => {
+        expect(() => {
+            Intl.getCanonicalLocales("en-t-en-POSIX-POSIX");
+        }).toThrowWithMessage(
+            RangeError,
+            "en-t-en-POSIX-POSIX is not a structurally valid language tag"
+        );
+    });
 });
 
 describe("normal behavior", () => {