In my experience, a lot of people tend to double-click web links, buttons, etc. when they only need to click once (they know they have to double-click everywhere else in Windows to open things, so the single-click web stuff is a bit of an exception to the rule and something that easily catches some people out).
For many sites, this means that the form is submitted twice, leading to double posts. I suspect that's all this is. Perhaps
@dave can modify the server-side code to check for duplicate submissions?
