In my book, double-posting when there is a compelling reason to do so (e.g., addressing disparate comments from multiple posters in digestible chunks, or remembering something relevant after the edit window has passed) is not a sin, nor a crime, nor even a minor party foul. Thus, it warrants no apology.
I feel the same way about very long posts. (Of course, authors should also be aware that such posts require more effort to digest than some readers are willing or able to invest, and they should consider whether their rhetorical goals are well-served by this strategy.)
Blatant thread hijacking is far more discourteous, as it is a greater hindrance to coherent, thoughtful discussion. There's room for everyone's pet topic -- but it's not often on someone else's thread on a different subject. Thread hijacking should generally be avoided, and when an apology is called for, it should be offered in as unobtrusive a way as possible.
While we're complaining about perceived BBS transgressions, I'd also like to register my extremely mild annoyance with posts announcing that a spammer has been reported (which I've seen at CHE and elsewhere). Their primary function seems to be to call attention to the person reporting the spam. Because reporting takes so little time, because reports about a given post are generally collated for the moderators,* and because most of us are aware enough not to be fooled by forum spam, there's really no need to alert others to the fact that a post was spam and has been reported. All such posts do is leave yet another bit of off-topic detritus in a (sometimes previously-dormant, but now needlessly resurrected) thread. Report the spam, but don't bother reporting that you've reported!
*At least, this is true on forums I've moderated; I do not know for sure if it is true on this platform and would welcome correction if I am wrong.
