This should be reasonably obvious on a Bitcoin forum I think. We're trusting immense amounts of wealth to a combination of 24 words. Even allowing for certain common language constructs, the chances of someone accidentally posting the same thing as someone else are... well, very low to say the least.

Having said that, there is a bigger issue here about prevention vs cure. If a simple sentence was added to the "Post reply" / "Start new topic" page next to the "Post" button, e.g. "don't make low value unnecessary posts and make sure to add links/quotes if you copied text written by someone else", I think that would significantly help users who don't really intend to plagiarize, but perhaps don't know (cultural differences and whatnot) that unreferenced copypasta is not cool, or forget to format it properly. It would also invalidate some common excuses that plagiarist use in ban appeals. This has been discussed ad nauseam and is a really minor tweak but unfortunately doesn't seem to be a priority for theymos.