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<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script>
document.addEventListener('DOMContentLoaded', function() {
let txt = new Set();
document.querySelectorAll('ul > li').forEach(li => {
txt.has(li.textContent) ? li.remove() : txt.add(li.textContent)
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<body>
<ul>
<li>1</li>
<li>2</li>
<li>1</li>
</ul>
</body>
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