Sales Order Line item

Hi Experts,

when we enter Material- 101 with a quantity of 1000 and by mistake if we enter the same Material- 101 again with quantity of 1000, It is appearing as another line item and the quantity shows 2000. My requirement is If we did this mistake ,we want all those quantities of one material should added up and show as one line item.How to control this, Please throw some light on this.

Thanks,

Kanna.