I have the same issue at
www.lagendijkmeubels.nl, using also update J.
I can add 2 different items, that`s no problem.
But when I add 1 product to the cart, and immediately after that want to add the same product again from the same page, is doesn`t work.
When I change the amount to 2, it works, but then I have 3 products in the cart. This is + the 1`st item, witch I can remove ofcourse .
So I have to remove first 1 article, and then I can add 2 of the same articles, instead of just adding one.