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+++ b/tests/wpt/web-platform-tests/css/cssom/CSSStyleSheet-constructable-duplicate.html
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+<!doctype html>
+<title>Cascade order of a stylesheet for duplicate sheets.</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://wicg.github.io/construct-stylesheets/">
+<script src = '/resources/testharness.js'></script>
+<script src = '/resources/testharnessreport.js'></script>
+<div></div>
+<script>
+test(function() {
+  let sheets = [];
+
+  for (let i = 0; i < 2; ++i) {
+    sheets.push(new CSSStyleSheet());
+    sheets[i].replaceSync("div { z-index: " + i + " }");
+  }
+
+  document.adoptedStyleSheets = [sheets[1], sheets[0], sheets[1]];
+  assert_equals(getComputedStyle(document.querySelector("div")).zIndex, "1", "duplicate stylesheet should take right position in the cascade");
+});
+</script>