title: check for duplicates when saving an appointment
body: <p>I would like to know how to prevent a user from double booking a therapist in the same timeslot that is already booked.</p>
  
  <p>Scenario:
  I have Amanda and Lisa. They both want to book Lorraine(Therapist) in the hour of 9am till 10am. Amanda Books Lorraine first at 9am to 10am. Lisa comes second and requests Lorraine from 9:30am till 10:30am on the same date as Amanda.</p>
  
  <p>Now how do I prevent that from happening since Lorraine is already booked in that hour. I am currently using dateTimePicker for my time </p>