diff --git a/lisp/org.el b/lisp/org.el
index e806440..fc850b8 100755
--- a/lisp/org.el
+++ b/lisp/org.el
@@ -15415,7 +15415,8 @@ Being in this list makes sure that they are offered for completion.")
 (defun org--update-property-plist (key val props)
   "Associate KEY to VAL in alist PROPS.
 Modifications are made by side-effect.  Return new alist."
-  (let* ((appending (string= (substring key -1) "+"))
+  (let* ((appending (and (string= (substring key -1) "+")
+                         (not (string= (substring key -2) "++"))))
 	 (key (if appending (substring key 0 -1) key))
 	 (old (assoc-string key props t)))
     (if (not old) (cons (cons key val) props)