partner-contact/partner_mobile_duplicate_warn/views/res_partner.xml

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

    <record id="view_partner_form" model="ir.ui.view">
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="base.view_partner_form" />
        <field name="arch" type="xml">
            <div
                attrs="{'invisible': [('same_vat_partner_id', '=', False)]}"
                position="after"
            >
                <div
                    class="alert alert-warning"
                    role="alert"
                    attrs="{'invisible': [('same_mobile_partner_id', '=', False)]}"
                >
                  Duplicate warning: partner <field
                        name="same_mobile_partner_id"
                    /> has the same <b>Mobile</b>.
                </div>
            </div>
        </field>
    </record>

</odoo>