OK, there is a lot of myths and truths floating around regarding duplicate content.
When Google came out with their updates targeting duplicate content the internet world went mad. Deleting anything that appears anywhere else on the web. Bad advice was thrown around and many webmasters slashed content from their site in case they got some duplicate content penalty.
Firstly let me tell you the good news, there is no such thing as a duplicate content penalty, this penalty is total a creation of people who think they know it all.
Google did, however, wage war against scrapper type sites. A site where all the content is scraped/stolen from other sites mostly to fool people into thinking their site is bigger than it really is.
We set about building a website built initially of duplicate content to see if it would rank. We signed up to quite a few affiliate programs including eBay and Amazon but also many others. We just displayed products and never wrote a single piece of content. The site ranked exceptionally well, so much so it was within just a few weeks achieving traffic like many well-established sites could only dream of.
So the short answer is if the majority of your site is unique and the content the users are posting is of benefit then fine, don't worry about.