@api.constrains('ref_ids', 'partner_id')
def _check_ref(self):
    for record in self:
        for ref in record.ref_ids.ids:
            bill = self.env['account.move'].search(
                [('ref_ids', 'in', ref), ('partner_id', '=', self.partner_id.id)])
            if len(bill) > 1:
                raise ValidationError(_("You can't create 2 bills with the same vendor and reference"))