I normally close the threads that come later. However, there have often been cases in which the original one has a very unclear subject line that makes it difficult to know that the topic already exists. As a result, there tend to be multiple duplicates because those posters don't realize what the original is about. In those cases, I may choose to lock the original and keep a thread with a more descriptive subject line. Also, in some case, the a duplicate thread may have a lot more replies than the original. In that case, it makes more sense to keep the duplicate.

I thought the first one was the one that was allowed to continue, but lately I have seen it done seemingly randomly.