Great question. In the past it hasn't been much of a problem, so I'm wondering if either the word hasn't spread about the contest, or if peeps are just drained on things to write about that aren't already covered.
Here's what we have in our term when people join BYC:
"You are granting us with a non-exclusive, permanent, irrevocable, unlimited license to use, publish, or re-publish your Content (including and not limited to posts, images, comments, articles, etc.) in connection with the Service. You...
Great question!
We require the articles to be unique. Posting to BYC grants us license to the content. We don't have any rules about you using the content in the future, but we probably should add that to future contest rules so there isn't question / issue with duplicate content, etc. (@sumi )