It often takes about 30 seconds for someone to become impatient and hit the button again. If the server is bogged down a little bit it can take a little over it for the post to actually be posted. That is where you get the double posts. You'll want to be sure your floodcheck time is set above the time it takes for your server to complete the process of making a post, then add a margin of error. 1 minute is usually just about right.