if anyone is interested in this, a simple defadvice appears to be a good
option, I put

(defadvice org-babel-execute-maybe (around org-babel-stop-on-collision)
  "stop execution of result file defined more than once"
  (let ((info (org-babel-get-src-block-info)))
    (setq result-file (cdr (assoc :file (nth 2 info))))
    (if (save-excursion
          (goto-char 0)
          (re-search-forward (concat ":file +" result-file) nil t)
          (re-search-forward (concat ":file +" result-file) nil t))
        (error (concat result-file " defined in more than one source
block"))
      ad-do-it)))
(ad-activate 'org-babel-execute-maybe)


in my .emacs and appear to get the desired functionality




2013/10/13 Samuel Wales <samologist@gmail.com>

> In case it helps, there is org-confirm-babel-evaluate.
>
> (But I have not found it to be useful, because it does not seem to
> place point in a place where you can check properties, etc.)
>
> Samuel
>
> --
> The Kafka Pandemic: http://thekafkapandemic.blogspot.com
>
> The disease DOES progress.  MANY people have died from it.  ANYBODY can
> get it.
>
> Denmark: free Karina Hansen NOW.
>