from another thread that I'm trying not to hijack:
The first click is when you clicked the first time ("I hit post once, just once...").
The second click is when you clicked on a dialog box that asks if you want to leave the page, so you click "yes."
That's two clicks. The problem here is that there is a disconnect between what the board is asking you, and what the board does in response to your click. You make a completely normal assumption that the board is going to do what it says it's going to do, so you provide the second click. Unfortunately the board has disconnected what it says it is going to do with what it actually ends up doing, because the php script that executes the board's action has been corrupted.
The key to not double posting in this situation is not to continue behaving in the same what that you've always behaved for years. IMO the key to success is to recognize that the board is misbehaving and to modify our behavior in recognition of the problem.
I see the same prompts that Enzo is seeing, but I don't click on any prompt that suggests proceeding with any subsequent action, the reason being that the action being associated with the button has become corrupted. Clicking on a second button is a necessary condition for the double post to occur.
I don't know if anyone else has encountered this problem, or if it's just Enzo and me. I had the double post problem for a while, but I've come to avoid it by not giving the board any commands that amount to approval of a proposed action after a failed post request. By doing that I'm avoiding the double post problem. YMMV.