I'm having an issue with cross sells being offered which are already in the cart. Let's say my store offers items A, B, and C. Item C is set up as a cross sell for both items A and B.

* I add item A to my cart. Item C now displays as a cross sell on the cart page, as it should.
* I click "add to cart" for item C. I now have A and C in my cart and no cross sell displayed. Everything is fine so far.
* Now I add item B to my cart. On the cart page I now see item C offered as a cross sell again even though it's already in my cart.

To further compound the situation, let's take another example: My store offers items A and B. A is a cross sell for B, and B is a cross sell for A.

* I add item A to my cart. Item B shows up as a cross sell.
* I click "add to cart" for item B. I now have A and B in my cart, and A is now showing as a cross sell.
* If I then click "add to cart" for item A (even though I already have one in my cart), I'll have A (twice) and B in my cart and B will be showing as a cross sell.
* I can repeat this cycle indefinitely.

Why is this happening? How can I change this behavior? What I need is for the cart to recognize that a particular cross sell is already in the cart and to keep that cross sell from displaying in that case.