How can duplicates be removed from a file using COBOL?

Finally it worked.

Here is the code:

   IDENTIFICATION DIVISION.
   PROGRAM-ID.RemoveDup2.
   ENVIRONMENT DIVISION.
   INPUT-OUTPUT SECTION.
   FILE-CONTROL.
   SELECT INPUTFILEDUP ASSIGN TO 'C:\Cobol\INPUTFILEDUP.txt'
           ORGANIZATION IS LINE SEQUENTIAL.
   SELECT OUTFILEDUP ASSIGN TO 'C:\Cobol\OUTFILEDUP.txt'
               ORGANIZATION IS LINE SEQUENTIAL.
   SELECT WorkFile ASSIGN TO "WORK.TMP".

   DATA DIVISION.

   FILE SECTION.
   FD INPUTFILEDUP.
   01 INPUTFILEDUPREC.
       88 EOFINPUTFILEDUP    VALUE HIGH-VALUES.
       02 INPUTFILEID        PIC 9(07).

   FD  OUTFILEDUP.
   01 OUTFILEDUPREC         PIC 9(07).

   SD WorkFile.
   01 WORKREC.
      02 WINPUTFILEID       PIC 9(07).

   WORKING-STORAGE SECTION.
   77 WS-VARIABLE            PIC 9(09) VALUE ZERO.
   77 REC-NOT-MATCH          PIC 9(01).
   77 CUR-VARIABLE           PIC 9(7) VALUE ZERO.

   PROCEDURE DIVISION.
   BEGIN.
       SORT WorkFile ON ASCENDING KEY WINPUTFILEID
       USING INPUTFILEDUP GIVING INPUTFILEDUP

   OPEN INPUT  INPUTFILEDUP
   OPEN OUTPUT OUTFILEDUP

       READ INPUTFILEDUP
               AT END SET EOFINPUTFILEDUP  TO TRUE
   END-READ
       PERFORM UNTIL (EOFINPUTFILEDUP)
           IF INPUTFILEID NOT EQUAL TO  WS-VARIABLE
                   MOVE  INPUTFILEID TO WS-VARIABLE
                   WRITE OUTFILEDUPREC  FROM  INPUTFILEID
                   READ  INPUTFILEDUP
                       AT END SET  EOFINPUTFILEDUP TO TRUE
       PERFORM UNTIL (EOFINPUTFILEDUP)
           ELSE
                   DISPLAY "DUPLICATE FOUND    "   INPUTFILEID

   READ INPUTFILEDUP
               AT END SET EOFINPUTFILEDUP  TO TRUE
   END-READ
   END-PERFORM

   CLOSE   INPUTFILEDUP
   CLOSE  OUTFILEDUP

   STOP RUN.