To decide whether to create a script based or front end solution, you first need to know if your results need to be selection sensitive or not. If script based, you need to decide if you want to create the count per class etc.
I don't get why Dick has a count of 2 in your sample data, while his ID 121 doesn't have any duplicates within this selection (Class = B).
Mayb along these lines:
LOAD Class, Name, ID, Date#(CreateDate,'YYYY-MMM-D') as CreateDate
Class,Name, ID, CreateDate
A,Jason, 121, 2013-Jan-1
A,Jason, 151, 2013-Feb-1
A,Dick, 121, 2013-Feb-2
B,Jason, 151, 2013-Mar-1
B,Dick, 121, 2013-May-1
B,Jason, 151, 2013-Jun-1
LOAD *, autonumber(CreateDate, ID)-1 as CountDupl
Resident TMP order by ID, CreateDate;
drop table TMP;