Requisition line convert to purchase line

hi,

   I am facing a problem when i create a requsition  from sale order, and calculate a plan(same item with same vendor ) then two or more line are created then click carryout action message then purchase order created with two item(Item is same with vendor) but client requirement is if item and vendor is same then create only one line for same vendor. i write a code of 333 codeunit (insert purchase header function) but same item cannot be acumalte with Quantity.  where exact location of write a code.(Please write a littile bit code).

Thanks

Sunil Mishra