Hello SuDarLy,

Put if condition before search any records because when form is load, onchange method is automatically called. Try below code.

class similar_contact(models.Model):

    _inherit= 'res.partner'

    @api.onchange("mobile")

    def onchange_mobile(self):

    if self.mobile:

        mobile_id=self.search([('mobile','=',self.mobile)])

        if mobile_id:

            raise ValidationError(_('Similar Mobile Number exist'))

        else:

            raise ValidationError(_('No Similar Mobile'))

Hope it will helps you.

Thanks,