>I'm not a dmoz expert but i think that most of the suggestion come from users
Well, now that you mention it, most suggestions do come from abusive spammers.
And certainly, some definitions of "user" (think psychology) would include such creatures.
On the other hand, from all accounts of people with DMOZ experience, most actual site listings probably come from somewhere than suggestions.
Which gets back to multiple listings per site. I've aggressively deeplinked a number of websites, and I have a list of websites (not suggested!) that should have more deep links. And the vast vast majority of THOSE listings CERTAINLY do NOT come from suggestions.
Why not? because (1) socially-adjusted, polite people follow the submittal policies, which means they DON'T suggest a site more than once or twice, and (2) the other folk, self-centered and rude, are generally not generating content of public interest.
In practice, this means that sites suggested multiple times are hundreds of times less likely to be listable at all anywhere, than sites only suggested once. In the extreme case, the strongest possible statistical proof that a site is NOT listable, is that it has been suggested dozens of times.