I wonder if this is browser specific. I typically INGO either from my mac or iPhone, or iPad. I've never had double posts, but occasionally, from the iPhone, after posting I've been sent to a page that says I have to wait at least 15 seconds after posting something to post again, or something like that.
So it looks like it tried to post double from my iPhone, but vbulletin detected the double post.
I am having this occur (the wait at least 15 seconds thing) from time to time on Firefox.