Fix duplication of labels definitions

parser.mly

@@ -13,7 +13,7 @@
     | [] -> [k]
     | k' :: l when k < k' -> k :: k' :: l
     | k' :: l when k = k' -> k :: l
-    | k' :: l -> k' :: k :: l
+    | k' :: l -> k' :: (insert1 k l)
 
   let label l =
     let l' = Label l in