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diff --git a/testing/web-platform/tests/custom-elements/v0/registering/definition-construction-algorithm-duplicate-definition.html b/testing/web-platform/tests/custom-elements/v0/registering/definition-construction-algorithm-duplicate-definition.html
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+++ b/testing/web-platform/tests/custom-elements/v0/registering/definition-construction-algorithm-duplicate-definition.html
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+<!DOCTYPE html>
+<html>
+<head>
+<title>Check duplicate definition</title>
+<meta name="author" title="Sergey G. Grekhov" href="mailto:sgrekhov@unipro.ru">
+<meta name="assert" content="If there already exists a definition with the same TYPE, set ERROR to DuplicateDefinition and stop">
+<link rel="help" href="http://www.w3.org/TR/custom-elements/#registering-custom-elements">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="../testcommon.js"></script>
+</head>
+<body>
+<div id="log"></div>
+<script>
+test(function() {
+    var doc = newHTMLDocument();
+    var name = 'x-a';
+    doc.registerElement(name);
+    assert_throws('NotSupportedError', function() {
+        doc.registerElement(name);
+    }, 'Exception should be thrown if definition with the same type already exists');
+}, 'Check duplicate definition');
+
+
+test(function() {
+    var doc = newHTMLDocument();
+    var name = 'x-b';
+    doc.registerElement(name);
+    HTML5_ELEMENTS.forEach(function(tagName) {
+        assert_throws('NotSupportedError', function() {
+            doc.registerElement(name, {
+                extends: tagName
+            });
+        }, 'Exception should be thrown if definition with the same type already exists');
+    });
+}, 'Check duplicate definition. Specify constructor');
+
+
+test(function() {
+    var doc = newHTMLDocument();
+    var name = 'x-c';
+    doc.registerElement(name, {
+        prototype: Object.create(HTMLAnchorElement.prototype),
+        extends: 'a'
+    });
+    HTML5_ELEMENTS.forEach(function(tagName) {
+        assert_throws('NotSupportedError', function() {
+            doc.registerElement(name, {
+                extends: tagName
+            });
+        }, 'Exception should be thrown if definition with the same type already exists');
+    });
+}, 'Check duplicate definition. Test different prototypes and extends');
+</script>
+</body>
+</html>