Surely it doesn't make sense to cross-sell a product with itself, does it?
At the 2010 Internet Retailer Conference and Exhibition, Jay Greenberg, VP of Ecommerce at Spencer's Gifts shared how he and his team A/B tested that very thing. It proved to lift conversion rates for products when they were shown in the product recommendations area.
Note that the cross-sells were labeled "People who liked this item ultimately bought," which is more relevant and persuasive than "Related Items" or "You might like," for which repeating the item might appear to be a mistake.
Dare to test this for yourself?