Hi,

You can avoid creating a duplicate contact by generating a error message by checking the unique field.

For example:

class ResPartner(models.Model):
    _inherit = 'res.partner'

 
    @api.onchange('name')
    def _check_name(self):
        partner = self.search([])
        name = partner.mapped('name')
        if self.name:
            if self.name in name:
                raise ValidationError('Duplicate Customer Name.')
 

Here, it will generate a validation error if the creating customer name already exists in contacts. And it will prevent creating customers with the same name.

Regards