web/web_tree_duplicate/templates/assets.xml

<?xml version="1.0" encoding="UTF-8"?>
<!-- Copyright 2019 Onestein
     License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl). -->

<odoo>
    <template id="assets_backend" inherit_id="web.assets_backend">
        <xpath expr=".">
            <script src="/web_tree_duplicate/static/src/js/backend.js"></script>
        </xpath>
    </template>
</odoo>