District Attendance Letters fail to generate with an error in CBS.STM - "These columns don't currently have unique values.<br>"

Cause

More than one student record with the same STUID number exists at a school  

Query to identify duplicate student records in a school:

SELECT SC, ID 
FROM STU 
WHERE DEL = 0
GROUP BY SC, ID
HAVING COUNT(ID) > 1

Resolution

Remove the duplicated student record