When creating the orderline check if there is already an orderline with the same fooditem.
TO do this retrieve the list of orderlines and with a list operation perform a find operation where you compare the currently selected fooditem name with the name in the list. Then if something is found, the item was already in an orderline so you'll need to change the amount to amount +1. If nothing was found from the find operation you need to create the orderline and set the amount to 1.