Right now the application shows a warning and adds duplicate item.

But I want to edit Sales quotation/order/delevery/invoice form so that it will not add similar item.

I want to show an error instead of warning and the "existing line containing the duplicate item will become editable(just like the line looks after edit button clicked)"

I have edited sales_order_ui.inc #51 to show error but i am unable to figure out how to make the duplicate line editable.

is there any function to fire the corresponding edit button? or any other suggestion?

Thanks
Ethan