Amusements

The following Emacs Lisp function duplicates the current line and comments the first line (using the prefix for the current mode) .

(defun upn-comment-and-duplicate ()
  "Comments current line after duplicating."
  (interactive)
  (let (
        (beg (line-beginning-position))
        (end (+ 1 (line-end-position))))
    (copy-region-as-kill beg end)
    (beginning-of-line)
    (yank)
    (comment-region beg end)))