When a second copy of an item is dropped onto my cart, I bump the count on the previous item and delete the new item.  The code that computes the cost of the cart then sees one item with a count of 2.

When another copy is dropped, the count on the item already in the cart is back to 1.  

Any idea why?

The code in question is in the dndDrop event handler:

    dojo.connect(cart, "onDndDrop", function(source, nodes, copy, target) {
       cartDrop(source, nodes);
       });
    dojo.connect(trash, "onDndDrop", function(source, nodes, copy, target) {
       cartDrop(source, nodes);
       });

cart, trash and added_id are globals.

function cartDrop(source, nodes) { 
    var total = 0;
    var cartItems = cart.getAllNodes();

    console.debug(ts()+"Checking for multiple "+added_id);
    // See if the item being added is already in the cart.
    var cartItems = cart.getAllNodes();
    var count    = 0;
    var not_the_first  = false;
    var first_index;
    console.debug("#cart items:"+cartItems.length);
    for (var i=0; i<cartItems.length; i++) {
       var id = cartItems[i].id;
       var item = cart.getItem(id);
       if (added_id == item.data.id) {
           count = count + item.data.count;
           console.debug("Cart["+id+"] count:"+item.data.count+"; total:"+count);
           if (not_the_first) {
              item.data.count = count;
              console.debug(item.data.id+"(index:"+i+", id:"+id+") count updated to "+item.data.count);
              cart.selectNone();
              cart.selection[cartItems[first_index].id] = 1;
              cart.deleteSelectedNodes();
              break;
              }
           else {
              not_the_first = true;
              first_index = i;
              }
           }
       }

    // Add up the prices in the cart:
    var cartItems2  = cart.getAllNodes();
    console.debug("[Computing cost] #cart items:"+cartItems2.length);
    for (var i=0; i<cartItems2.length;i++) {
       var id = cartItems2[i].id;
       var item2 = cart.getItem(id);
       console.debug("CartItem "+i+"; id:"+id+" count:"+item2.data.count+" price:"+item2.data.price);
       total += parseFloat(item2.data.price) * item2.data.count;
       }
    dojo.byId("cost").innerHTML = dojo.currency.format(total, {currency: "USD"});
    };

Annotated logfile:

When the second item is added to the cart, the adding up code sees the count is two:
[Computing cost] #cart items:1CartItem 0; id:dojoUnique10 count:2 price:1.50

When the third item identical is added, the count for dojoUnique10 has gone back to 1:
19:26 Checking for multiple S002
#cart items:2
Cart[dojoUnique10] count:1; total:1
Cart[dojoUnique12] count:1; total:2