Or HaChaim on Exodus 8:17/אור החיים על שמות ח׳:י״ז

1 ומלאו בתי מצרים את הערוב, the houses of Egypt will be filled with wild beasts, etc. The reason the Torah repeats the word הערוב, "the wild beasts," is that the houses will be filled with the wild beasts; if the Torah had merely written once: "the wild beasts will fill the houses of Egypt," we would have thought that the purpose of the plague was simply to fill the Egyptians' houses with wild beasts; this was not so, however, as the Torah had already stated: "I will send the wild beasts against you, your servants, your people as well as against your houses;" the obvious meaning is that the beasts were to cause damage and destruction. As it is, the additional message is that the houses would literally be filled with wild beasts.

2Another meaning of the words את הערוב the second time could be that had the Torah not repeated these words we would have assumed that the plague would have exhausted its purpose as soon as the houses of the Egyptians had become full of wild beasts; however, this was not the full extent of the plague. There were so many wild beasts that they could not be accomodated even after they had already filled all the houses of the Egyptians. This is why the Torah added: "and also the ground whereon they are."