added popup saying multiple copies of react detected

Author: chisler

shells/webextension/popups/duplicated.html

@@ -0,0 +1,21 @@
+<script src="shared.js"></script>
+<style>
+    html, body {
+        font-size: 14px;
+        min-width: 430px;
+        min-height: 101px;
+    }
+</style>
+<p>
+    <b>This page is using multiple copies of React. &#x1f6a7;</b>
+</p>
+<p>
+    Note that using several copies of React may cause problems.
+    <br />
+    <br />
+    You can find the troubleshooting instructions in <a href="https://fb.me/react-refs-must-have-owner#multiple-copies-of-react">React Blog</a>.
+</p>
+<hr />
+<p>
+    Open the developer tools, and the React tab will appear to the right.
+</p>