Musar zu Tehillim 78:36

It is quite appropriate then for Rabbi Shimon ben Lakish to say that Exodus 7,3 can be used to refute the arguments of the heretics. He takes his cue from the word בפיהם, people whose repentance is only "mouth-deep," does not involve their heart. Pharaoh was a perfect example of such a person. This is meant by "the addition of impurity to already existing impurity." Rashi refers to the additional refusals of Pharaoh to repent sincerely after he had failed to repent during the first five plagues. From that time on the impurity remained anchored in his heart, i.e. invisible to outsiders. G–d considered it necessary to demonstrate to outsiders the true state of Pharaoh's mind, therefore He made sure that Pharaoh would display universally visible signs of his impurity.