You'd have to prove that you did formulate it independently yourself. There couldn't be other prior art. If your own inventor's journal revealed a large amount of work, that would show the original patent was indeed valid. If you could prove that you did invent it yourself, and that it wasn't that big a deal, the patent should fall.

So to win in your case, you'd have to tread a tightrope between "not enough evidence to show independent origin" and "the thing is obviously substantive enough to warrant a patent."

Most likely, the thing actually is obvious and there would be other prior art, which the Patent Office currently doesn't pay attention to.