I've never got to the bottom of that one. It's either entirely on Eternal- September's side (I never saw it happen when I used to use news.individual.net), or it's an interaction between E-S and my news client (E-S incorrectly reports a post as having failed and so my client automatically re-posts, say). I've heard of it happening to other people using E-S, too.
I suspect the only way to get to the bottom of it is to tweak the source for my news client so that it logs all NNTP traffic - but even if I can find out what's going on, it doesn't necessarily mean I'll be able to fix it...
cheers
Jules