Looks like our spam prevention plugins are acting a bit wonky and forcing people to submit their comments twice (and giving a cryptic error message inbetween submits to boot). I don't know how to fix the plug-in; I'll leave that to Matthew. But since the plug-in was essentially forcing commenters to preview their comments, I decided to remove the confusion by removing the "Post" button the first time around.
Same rigamarole, hopefully less confusion.
And hopefully Matthew can fix our plug-in in the near future.