How to catch duplicate comments
I'm not aware of any module that would provide this particular functionality. First of all, I would reduce the risk for confusion. An example is: Store a CID in the users $_SESSION, and theme the comment form with "You have X comments awaiting moderation."

You could also add a custom validation function to the comment form with a hook_form_ID_alter, and simply deny comments that fall within your stated condition, "Same body text within 5 minutes from same session", or so.

I would definitely focus on the first one though. No matter what type of validation criteria you add, there will always be edge cases where there is a good reason for posting something that your will then reject. This will of-course be a tradeoff between how large a risk you are willing to take rejecting valid input, and the effor required to "un-confuse" users.