I've Made A jQuery AJAX Chat, Sometimes Double Posts?

[Alex Price]

Obviously, it uses a timestamp to fetch new messages, but sometimes when hte user has posted something, it adds the message to the chat's view twice. When you refresh the page, it's only there once, but it seems to add the new message that the user sent twice sometimes.

Any idea why?

Oh and does anyone know any good tutorials on making an advanced chat with PHP, jQuery and AJAX? Mine at the moment is only a basic one that polls the server every 2 seconds.