Oh, you mean in case the flag had got corrupted in the DB? Tried it, but still getting two posts.
I mean, we're talking about a flag set for 15 (now 30) seconds, and a double-click speed of between 80 and 100ms.
My server seems to respond to a posting within 300ms, therefore, I wonder if there's some kind of queue/race condition whereby it is not able to check the DB for previous posts as fast as the second post arrives? Which is why I think something "front end" might be better.
Or, as a last resort, possibly there might be something in nginx to prevent rapid fire "post" submissions. But it would be good to fix this natively if at all possible