On the calendar once you have selected a session you will see a little doctor icon appear in the bottom right hand corner of the date cell.
Clicking the doctor icon turns it green. This means that a duty doctor session will be on that day.
You can mark a Duty Doctor as often as required.
When you come to make a booking, if the session needed cover is all day, you will have the option of selecting whether this duty doctor will be a morning session, afternoon session or both.
If the session you have selected is AM, or PM and then marked it as a Duty Doctor, when it comes to making the booking, its assumed that the AM, or PM session will be a Duty Doctor.