(defadvice narrow-to-region  (around clone-when-narrowing-already-visible-buffer activate)
  "Clone an indirect buffer when attempting to narrow the
contents of buffer visible in more than one window."
  (save-excursion
    (when (> (length (get-buffer-window-list (current-buffer) nil nil)) 1)
      (let ((cloned-buffer (clone-indirect-buffer nil t) ))
        (switch-to-buffer cloned-buffer t)))
    ad-do-it))