Hi,

This happens because the event registration flow in Odoo doesn’t always merge or detect existing contacts.

To fix or mitigate this:

    Customize registration logic

    If possible, override or extend the registration process so it does something like:

    existing = env['res.partner'].search([('email', '=', registration_email)], limit=1)

    if existing:

        use this partner

    else:

        create new partner

    Use / check OCA module for “Unique Partner per Event”

    There is an OCA module that prevents duplicate partner registration per event (ensures same partner is reused). Odoo Community Association (OCA)

Hope it helps