novice: table design FOREIGN key

Hi All Thanks for all your comments on my previous mail, very much appreciated. I'm stuck again! I've created the following lookup table for m <-> m relationship between a Group and a Contact table. CREATE TABLE Group_Contact( GroupID INT NOT NULL, ContactID INT NOT NULL, Index (GroupID), FOREIGN KEY (GroupID) REFERENCES Group (PKey) ON DELETE CASCADE, Index (ContactID), FOREIGN KEY (ContactID) REFERENCES Contact (PKey) ON DELETE CASCADE ) TYPE=InnoDB COMMENT='Group to Contact lookup'; what do I need to do to ensure only unique (GroupID,ContactID) pair can be inserted into the table? Thanks Tony Yau