Duplicated string literals make the process of refactoring error-prone, since you must be sure to update all occurrences.
On the other hand, constants can be referenced from many places, but only need to be updated in a single place.
Noncompliant code example
With the default threshold of 3:
PROCEDURE DIVISION.
DISPLAY "Firstname: ".
*...
DISPLAY "Firstname: ".
*...
DISPLAY "Firstname: ".
Compliant solution
WORKING-STORAGE SECTION.
01 FIRST-NAME-HEADER PIC X(42) VALUE "Firstname: ".
PROCEDURE DIVISION.
DISPLAY FIRST-NAME-HEADER
*...
DISPLAY FIRST-NAME-HEADER
*...
DISPLAY FIRST-NAME-HEADER
Exceptions
Literals with fewer than 7 characters are ignored.
Only duplications located in a PROCEDURE DIVISION
, not those contained in copybooks are reported.