重复字符保留问题

create cursor b1 (m c(5),m1 c(3))
insert into b1 values ("1002","")
insert into b1 values ("12123","")
insert into b1 values ("123","")
insert into b1 values ("01110","")
insert into b1 values ("22201","")
insert into b1 values ("02002","")
cm = ""
nm = 0
ch = ""
ret = ""
REPLACE ALL m1 WITH fun()
SELECT * FROM b1
RETURN

FUNCTION fun()
    cm = ALLTRIM(m)
    nm = LEN(m)
    ret = ""
    FOR i=1 TO nm-1
        ch = SUBSTR(cm,i,1)
        FOR j=i+1 TO nm
            IF SUBSTR(cm,j,1)==ch
                ret = ret + ch
                EXIT
            ENDIF
        ENDFOR
    ENDFOR
    RETURN ret
ENDFUNCv