There's been a rash of double posts lately, and having experienced one myself yesterday, I am now suspecting that the Javascript or whatever code it is that is supposed to detect a duplicate post within a certain period of time (or heck, any post at all by the same poster) is broken or missing.
It used to be that if we accidentally double clicked, we'd get a warning popup that the forum system had detected an identical post from us in the last minute, or whatever.
That does not seem to be the case any more.