Hi Vinod ,
Please read the below statements , I think your problem will be solved.
Difference bt MODIFY & UPDATE
Modify:
If the value of key fields in the internal table matches with the key fields values in database table,modify will update the existing entry.Otherwise,it will insert new entry.
Update:
To update db table,it is mandatory that the value of key fields in internal table should match with database table.
As you are using MODIFY statement , read which doesn't matches the key fields in the db table and getting created a new entry
Thanks,
S.Rajendranath Raparthi.