diff --git a/Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.cpp b/Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.cpp
index 0cc850bd00..7d8fb3ca2a 100644
--- a/Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.cpp
@@ -26,7 +26,7 @@ static Optional<Unicode::LocaleID> is_structurally_valid_language_tag(StringView
         quick_sort(variants);
 
         for (size_t i = 0; i < variants.size() - 1; ++i) {
-            if (variants[i] == variants[i + 1])
+            if (variants[i].equals_ignoring_case(variants[i + 1]))
                 return true;
         }
 
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Intl/Intl.getCanonicalLocales.js b/Userland/Libraries/LibJS/Tests/builtins/Intl/Intl.getCanonicalLocales.js
index ba988aa2d0..4164897e23 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Intl/Intl.getCanonicalLocales.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Intl/Intl.getCanonicalLocales.js
@@ -18,6 +18,13 @@ describe("errors", () => {
     });
 
     test("duplicate variant subtags", () => {
+        expect(() => {
+            Intl.getCanonicalLocales("en-posix-POSIX");
+        }).toThrowWithMessage(
+            RangeError,
+            "en-posix-POSIX is not a structurally valid language tag"
+        );
+
         expect(() => {
             Intl.getCanonicalLocales("en-POSIX-POSIX");
         }).toThrowWithMessage(