Many smart merchants provide affiliates with content in the form of articles, copy and reviews. I'm sure some of the merchants that provide the content and the affiliates that end up using it, wonder about duplicate content filters and potential penalties. The Get Elastic eCommerce blog has a some detailed insight and suggestions.
Dodging Duplicate Content Filters While Assisting Affiliates "You as a retailer with an affiliate program may wish to provide content for affiliates such as expert / editorial product reviews, general advice / guides on the product or activities related to the product or even product description content itself. This is great affiliate nurturing on the part of your affiliate management team - but it’s vulnerable to duplicate content filtering."
Another related GetElastic article that deals with duplicate content and affiliate links that knock the merchants out of the serps and replace their rankings can be found here:
Tips for SEO Friendly Affiliate Programs