Or HaChaim on Exodus 9:34/אור החיים על שמות ט׳:ל״ד

1 וירא פרעה כי חדל המטר, Pharaoh saw that the rain had stopped, etc. The Torah mentions this to show that Pharaoh's repentance already evaporated; he continued to sin almost immediately. When the Torah reports him as ויוסף לחטא, "he continued to sin," this does not refer to his continuing on his previous path; it means that Pharaoh added additional sins to the ones he had committed already. He now made a commitment not to release the Israelites under any terms and conditions.