I've always assumed it was deliberately left porous to compensate for the luck of the draw. There's a lot of randomness in which articles get an initial boost on /newest. If the duplicate detector were airtight, a lot of good stories would die unseen. This way, they can have another shot at a discussion.

Obviously one should not abuse this to post things that have already had a good discussion or (worse) to promote uninteresting content.

I saw jamesbritt's submission a few days ago—it may even have been how I learned about the article. I thought it was a shame that it fell through the cracks, hence the repost.