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)