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diff --git a/js/src/tests/non262/Intl/duplicate-variants.js b/js/src/tests/non262/Intl/duplicate-variants.js
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+++ b/js/src/tests/non262/Intl/duplicate-variants.js
@@ -0,0 +1,46 @@
+// |reftest| skip-if(!this.hasOwnProperty("Intl"))
+
+// RFC 5646 section 2.1
+// variant       = 5*8alphanum         ; registered variants
+//               / (DIGIT 3alphanum)
+
+// Duplicate variants are forbidden.
+assertEqArray(Intl.getCanonicalLocales("de-1996"), ["de-1996"]);
+assertThrowsInstanceOf(() => Intl.getCanonicalLocales("de-1996-1996"), RangeError);
+
+// Multiple different variants are allowed.
+assertEqArray(Intl.getCanonicalLocales("sl-rozaj-solba"), ["sl-rozaj-solba"]);
+
+// Variants can have the same prefix.
+assertEqArray(Intl.getCanonicalLocales("zh-Latn-pinyin-pinyin2"), ["zh-Latn-pinyin-pinyin2"]);
+
+// Values in extension sequences are not counted as variants.
+assertEqArray(Intl.getCanonicalLocales("en-u-kf-false-kn-false"), ["en-u-kf-false-kn-false"]);
+
+// Also test duplicates in Unicode extension keywords and attributes.
+// From https://tools.ietf.org/html/rfc6067#section-2.1
+//
+//    An 'attribute' is a subtag with a length of three to eight
+//    characters following the singleton and preceding any 'keyword'
+//    sequences.  No attributes were defined at the time of this
+//    document's publication.
+//
+//    A 'keyword' is a sequence of subtags consisting of a 'key' subtag,
+//    followed by zero or more 'type' subtags (so a 'key' might appear
+//    alone and not be accompanied by a 'type' subtag).  A 'key' MUST
+//    NOT appear more than once in a language tag's extension string.
+//
+//    ...
+//
+//    Only the first occurrence of an attribute or key conveys meaning in a
+//    language tag.  When interpreting tags containing the Unicode locale
+//    extension, duplicate attributes or keywords are ignored in the
+//    following way: ignore any attribute that has already appeared in the
+//    tag and ignore any keyword whose key has already occurred in the tag.
+//
+// The duplicates itself are removed in CanonicalizeUnicodeLocaleId, step 2-3.
+assertEqArray(Intl.getCanonicalLocales("en-u-kn-false-kn-false"), ["en-u-kn-false"]);
+assertEqArray(Intl.getCanonicalLocales("en-u-attr1-attr2-attr2"), ["en-u-attr1-attr2"]);
+
+if (typeof reportCompare === "function")
+    reportCompare(0, 0);