partner-contact/partner_email_duplicate_warn/views/res_partner.xml

<?xml version="1.0" encoding="utf-8" ?>
<flectra>

    <record id="view_partner_form_with_email_warning" model="ir.ui.view">
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_form" />
        <field name="arch" type="xml">
            <xpath expr="//div[@name='warning_company']" position="after">
                <div
                    class="alert alert-warning"
                    role="alert"
                    name="warn_duplicate_email"
                    invisible="not same_email_partner_id"
                >
                        Duplicate warning: partner <field
                        name="same_email_partner_id"
                    /> has the same <b>Email</b>.
                </div>
            </xpath>
        </field>
    </record>

</flectra>