In the following example, compilation would result in two classes named grp1. To fix the error, rename one of the 01 level items.
$set ilsmartlinkage
 program-id a.
 linkage section.
 01 grp1.  *> rename one of these items
    03 nam pic x(20).
 procedure division using grp1. 
  ...
 end program.
 program-id b.
 linkage section.
 01 grp1.  *> rename one of these items
    03 num pic 9(9).
 procedure division using grp1.
  ...
 end program.