用户登录或退出时限制购物车中的产品数量

Question

I have successfully limited the quantity amount of a single produt in the shopping cart (that is one). But there is a problem.

1. Logged in, the user adds the item to the cart.
2. The user logs off.
3. The user adds the same product to the cart when logged out.

4. The user logs in.

   Then the user still gets the "Exceded product quantity", but the product is updated to two at the cart.

   Is this a bug? Are there any workarounds, please? Thanks.

F.