That's the right answer, conceptually, but in practice it ain't that simple, because the Web protocols are stateless and naturally multitasking. The first time you post a message, the server starts some software to receive; and every time you post another copy, it starts a fresh software process. The Nth hand has no idea what the N+1th hand is doing.

It isn't impossible to prevent multiple posts, just a non-trivial problem; and it's long been on my list of "really really nice features to have". Someday....

Don't take the "human error" thing too personally. The root of the crosswalk-button phenomenon is an unwillingness to trust technology (which is not at all irrational, especially for those of us who know it too well) and a very human desire to give it "encouragement" by poking at it. The trick is to trust the system to let you know if your message didn't make it, then try again. You'll see an error message if the server choked, or a message from your browser if it couldn't get the message to the server. It won't leave you guessing.

--Robert