Fee Letters - Why are there duplicate letters printing for the same Student?

When a user runs the Fee Letters, that will generate Fee Letters to both the Parent/Guardian from Demographics, and from Contact records that have the Mail Tag. The Mail Tag field is to be used for a non-custodial parent/guardian who has a different address than the student's Mailing Address. The user can run a query to check for the Mail Tags, and compare the student's Mailing Address with the Contact's address.  

LIST STU CON STU.ID STU.NM STU.AD CON.NM CON.AD CON.MT IF CON.MT # " "  

If the addresses match, then the Mail Tag needs to be removed from the Contact record.