yeah i posted something this morning (about 12 hours ago) and i can confirm it double-posted. like you described, first posted with an error message (but it worked), then posted again with success message.

the weird thing is this:

the first post only included a link to the image, while the second post (which was posted in exactly the same way, just by clicking "create [post]" again) includes the image in a proper way. which makes previewing nicer. that's weird, right? i like the second type more, btw.