Hi Doo,
Please use following code to duplicate last item in your list component and set certain text to new item:
pre
//Note you need replace "mobilelistitem_42" with your list item component name.
var items = jQuery('[name="mobilelistitem_42"]:not([_tmpl="true"])');
var itemToDuplicate = jQuery(items[items.length - 1]);
//Duplicate an item.
var clone = itemToDuplicate.clone();
//Set text to new item
clone.find("h3").text("New text you need");
//Attach item in the list(after last item);
clone.insertAfter(itemToDuplicate);
/pre
Note please read comments in the code above.
Regards.