if doctor as 5 existing appointment no furter appointments should be booked

there is the email field if one email record is created in that field ,when the same mail is repeated again it should not saved

(validation rules ) I have an status (picklistfiled) 1)status is closed 2)once the record id created ,then dont rewrite the status value

if doctor as 5 existing appointment no furter appointments should be booked

there is the email field if one email record is created in that field ,when the same mail is repeated again it should not saved