This works when there is an active region, but fails to leave the selection when it defaults to using the whole line.

keybindings.el

;; Vim's o and O
(defun jeg2s-newline-below (skip-eol)
  "Insert a new line below the current line and indent it."
  (interactive "P")
  (unless (or (eolp) skip-eol)
    (end-of-line))
  (newline-and-indent))
(global-set-key (kbd "C-c o") 'jeg2s-newline-below)
(defun jeg2s-newline-above ()
  "Insert a new line above the current line and indent it."
  (interactive)
  (unless (bolp)
    (beginning-of-line))
  (newline)
  (previous-line)
  (indent-according-to-mode))
(global-set-key (kbd "C-c O") 'jeg2s-newline-above)

;; TextMate's C-D
(defun jeg2s-duplicate-line-or-region ()
  "Duplicate the current region, or line, and leave it selected."
  (interactive)
  (unless (region-active-p)
    (if (= (line-number-at-pos) (count-lines (point-min) (point-max)))
        (progn (call-interactively 'jeg2s-newline-below)
               (previous-line)))
    (unless (bolp)
      (beginning-of-line))
    (call-interactively 'set-mark-command)
    (next-line))
  (call-interactively 'kill-region)
  (yank)
  (yank)
  (setq deactivate-mark nil)
  (activate-mark))
(global-set-key (kbd "C-c d") 'jeg2s-duplicate-line-or-region)