My guess is that you have your mouse button interface set too sensitive. You can fix this by going into Start Button/Settings/Control Panel/Mouse and playing around with the double-click-sensitivity slider. (I'm presuming that your operating system is Windows. If not, well, THERE'S your problem!)
But, the NA website's user interface is partially complicit, too- it wouldn't be too difficult to add a "click filter" in the user interface that would reject multiple "Submit Follow Up" box selects within a reasonable period of time- say, a couple of seconds. That would ensure that someone's first click would be posted and subsequent, probably erroneous duplicate ones, from the same poster would be rejected if they happened within a short span of time after the first one. That would spare everyone the heartbreak of having multiple posts (which I think everyone who uses the Forum has fallen prey to at some time or other).
On the other hand, who really cares?