Comment and duplicate line

gistfile1.el

(defun comment-and-duplicate-line ()
  "Copy current line to line below and comment current line."
  (interactive)
  (let* ((beg (line-beginning-position))
         (end (line-end-position))
         (line (buffer-substring-no-properties beg end))
         (column (current-column)))
      (comment-region beg end)
      (goto-char (line-end-position))
      (newline)
      (insert line)
      (move-to-column column)))

(global-set-key (kbd "C-c C-d") 'comment-and-duplicate-line)