Sum of quantity if a item is selected twice on the sales order item grid.

Hi Everyone,

I am new to boyum usability and working on a line loop function where i am stuck with a case.

Function is on sales order add mode and i am trying to sum the quantity if an item is selected twice on the grid and store the sum of quantity to @store1.

Sort of a Cumulation of quantity to be store on the line level field.

Can any expert help me with this?