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You should have a policy of requesting that they quote material from other sources, and include a link back to the source. Aside from that, just be happy you have an engaged and active userbase. For stuff like press releases, have a no PR policy except in specific subforums. I wouldn't worry a ton about the duplicate content factor unless you expect this to be a huge amount of your content. Even then, if the community is producing lots of other original content, especially in the same thread as the duplicate content, then a certain amount of this type of content should be OK in my opinion.