Unnecessary duplicate checks in add Patient/Doctor/Appointment commands

[derekjxtan]

All 3 commands calls has patient/doctor/appointment before adding, but duplicate checks are already handled by UniqueItemList. To be completed:

1. Update UniqueItemList methods to throw the necessary exceptions
2. Update the methods (in Database, NewModelManager, etc.) that calls UniqueItemList methods to catch/throw the exceptions
3. Remove the unnecessary checks