How can we make the database ensure each record is unique based on the values from several fields?

There is a table containing course surveys. The fields are ResponderName, CourseName, Semester, plus survey answers. In other words, the primary key is made up of ResponderName, CourseName and Semester.

Say we have the following records:

record 1

ResponderName: John

CourseName: econ101

Semester: fall

record 2

ResponderName: John

CourseName: chemistry101

Semester: fall

How can the database be made to prevent the entry of a third record in which the primary key is a duplicate of one of the first two?

Thanks.

See if this helps:

http://fmforums.com/forum/showpost.php?post/204677/