Following up the posts on “This name has won a previous contest (so you can’t submit it)”, I also find it very odd if the submissions are in a totally different business field. Even the Trademarks office allows the same name to be trademarked multiple times as long as the class/goods & services are different. Think of “Delta”, trademarked for both an airline and kitchen tap manufacturer.
A related comment is that several times I have checked on some of the “preferred naming style” lifestyle images at the bottom of a contest page, only to find the domain has been dropped and is now available. I would love to be able to submit some of these but of course I get the “previously won” message.