Sorry for that last part being so short. For the unique id I'm just referring to the item id that you assign in the add to cart server behavior. The item's id will need to be unique to that item only. Each item that you have an add to cart for should have a unique id so this is no different than the other items.
Once you have the add to cart on the page you can update the trigger for the add to cart, it might look like this by default:
// WA eCart AddToCart
if (isset($_POST["WA_Store_Cart_1_ATC"]) || isset($_POST["WA_Store_Cart_1_ATC_x"])) {
You will need to update it with the eCart code that will check for the specific item in the cart, this check is based on the item id. In this example the item has an id of 5.
// WA eCart AddToCart
if ($WA_Store_Cart->InCart("5") != true) {
This code is saying 'if item 5 is not in the cart then proceed'. The rest of the code is just the regular add to cart code so it will just add this item to the cart. In this code it works like this:
this part represents the name of your cart
$WA_Store_Cart
This part is checking for the item
->InCart("5")
This final part just says 'if not true' meaning the item is not in the cart
!= true
