I believe what stops people from duplicating most of the time is those "Oops, you've been directed here from the wrong location" errors, if Neo has failed to have them up on a certain part of the site then it would be easy enough to abuse anything using that method to duplicate.
I think one of the main things is people don't know how this "Oops, you've been directed here from the wrong location" works (afterall, would you want to experiement on it? It's going to get very very cold!) so people don't know how easy/hard it is to manipulate using some form of coding on the page you're coming from and people don't know if maybe this page glitches every once-in-awhile (like the rest of the site).
In inclined to agree with Alina on this one, I doubt that the people have actually been able to hack/crack and override the servers and scripting due to the levels of security they have protecting the site as a whole. I think its more likely they manipulated a small flaw while they've been on the site which I'd see as being closer to abusing a glitch (the only difference being is that not every Neopian and his Lupe can abuse this glitch).
Nonetheless, I don't doubt that TNT has fixed this problem so it won't happen again
