Why duplicates in my_SELECT statement-Small issue in SQL

Question

Hi Experts,
I hv follwoing SQL, but, in some cases, am getting duplicates......any clue? is it bcoz, am using BSIS first and then BKPF(Header) as second?
SELECT s~hkont
         s~belnr
         s~bschl
         s~shkzg
         s~dmbtr
         s~kostl
         s~prctr
         k~bukrs
         k~gjahr
         k~monat
         k~belnr
         m~saknr
         m~xbilk
         m~gvtyp
    INTO TABLE t_main
     FROM bsis AS s
      INNER JOIN bkpf AS k ON kbukrs = sbukrs AND
                            kbelnr = sbelnr AND
                            kgjahr = sgjahr
      INNER JOIN ska1 AS m ON msaknr = shkont
         WHERE s~hkont   IN s_hkont AND
               s~kostl   IN s_kostl AND
               s~prctr   IN s_prctr AND
               k~bukrs   IN s_bukrs AND
               k~gjahr   IN s_gjahr AND
               k~monat   IN s_monat.

Answer 1

thanq.
After implememnting the Alvaro Tejada suggestion i.e. Using the DISTINCT clause, I moved the prog. into TEST/QA systems.
So, pls. let me know that,
1 - Is that suggestion(DISTINCT) wuld NOT work in all cases, like, some times may yield incorrect data?
2 - If so, will implement ur suggestion, i.e. By using T001 table, will JOIN the SKA1 table.......but, here making one more transport is tedious with a bit paper work.......so, if u say, its must, will create a new transport!
Its friednly asking, not criticing
thanq