if you post the identical post in the same thread twice within 5 mins it tells you: "that's the same post you made in the last 5 mins. Do you really wanna post it?"
but the 30sec notification means you have to wait till you are "unblocked" for posting again. The post isn't done then.
maybe it's got something to do with servers, browsers...
when I got the 30sec I don't post again. on checking later, my post is always there