I suspect that duplicate submissions are, to a considerable extent at least, are caused by people accidentally clicking on Submit twice, or two posts requests otherwise being generated and sent. A simple solution for this would be, I think, to add a hidden input field to the form containing a randomly-generated value that would then be used to determine whether that form had been submitted already or not. A hash function like MD5 or SHA-1 could (and should) be used for this; that way, it would be reasonable to assume at the very least that there would be no false rejections.

The advantage of this approach would be that it would actually be possible to post nodes with the same title in rapid succession - this could very well be needed with comments, too, where node titles like "Re: Re: some question" could well belong to nodes with different contents even if they are posted only 47 seconds or so apart from each other.

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mowgli