Tuple table update issue..........

I have a Tuple table in a main table with 2 colums: material number & quantity, after the intial full load when ever the quantity for a particular material changes and when we get delta load, instead of updating the quantity in tuple table, its writing new row in the list ie now we are having 2 records with same material number but different quantity, what should i do inorder to update quantity instead of writing a new record for same material number..., did any one faced this issue, please guide me ..........