Site owners and merchants should be concerned when duplicate content starts to negatively affect site rankings. 

The AMWSO blog has a great post outlining the issues and possible fixes for duplicate content in the context of affiliate marketing…

Many merchants who run affiliate programs make available content for their affiliates to use on affiliate sites. A common question / concern asked of AMWSO is if this content is being distributed to all affiliates, and multiple affiliates are using the content, will this incur a search engine penalty?  It’s a question asked of us both from the merchant perspective, as they redistribute content on their own pages, and also from the affiliate perspective, since the affiliates want to use that content.

Affiliate Marketing Blog by AMWSO: Duplicate Content Search Engine Penalties and Affiliate Marketing - Affiliate Marketing Blog