org.el: Ensure `org-drawers' doesn't contain duplicates.

* org.el (org-set-regexps-and-options): Ensure `org-drawers'
doesn't contain duplicates.

Thanks to Achim Gratz for spotting this problem.

lisp/org.el

@@ -4547,7 +4547,7 @@ but the stars and the body are.")
 			      (mapcar (lambda (x) (org-split-string x ":"))
 				      (org-split-string value)))))))
 	     ((equal key "DRAWERS")
-	      (setq drawers (append org-drawers (org-split-string value splitre))))
+	      (setq drawers (delete-dups (append org-drawers (org-split-string value splitre)))))
 	     ((equal key "CONSTANTS")
 	      (setq const (append const (org-split-string value splitre))))
 	     ((equal key "STARTUP")