Here are my own understandings of the exceptions for double-posting. As I am aware, there are only really two exceptions to this.
If adding something new to your work (Whether it be your fan-fiction) Or adding a new piece of information to a Thread, if you think that it is relevant. The first exception would be for your own work. As anything you needed to say again, could be done so in an edit. And/or brought to the attention of the creator of the Thread via a Profile Message of a Private Message. The second exception could apply to your own Thread, or somebody else's.
However, in the case for exceptions, double-posting would be considered bad if it is soulfully done to bump a dead Thread. Be the Thread your own, or anyone else's.
That is all that I am aware of in the case of exceptions for double-posting. Primarily it is against forum rules. And though there may be exceptions, I wouldn't suggest consistent use of these exceptions.