Checking for Duplicate Records

Having issues thinking the logic over this.

TableA: Personnel (Primary Key) PIdNum
TableB: Classes (Primary Key) ID, (Foreign Key) PIdNum, (class attened) InstClass
- One to many Relationship (1 person may have many classes)

Problem:
- I dont want Person A to have the same class entered any more than 1 time!

How can I check the table to see if a new record (dancing class) has already been entered for Person A. Cannot make InstClass a Primary key.

- Check with DAO?

Peter