Not sure what that "normal rate" of Round-up would be, as the label recommends multiple concentrations depending on your purpose. But, if you mean twice the highest rate, you're burning the foliage before the maximum absorption of the chemical be be achieved. In other words, the weed is not taking in the poison as much as it should. You're doing a good job killing surface tissue, but not so good at killing the whole plant. You're wasting money and poisoning the environment way more than you need, too.
