Theophylact of Ohrid

As for whether there was one servant girl who accused Peter, or yet another — Matthew says there was another, while Mark says that the same servant girl accused Peter both times. But this causes us no difficulty whatsoever regarding the truth of the Gospel. For here the disagreement of the Evangelists does not concern anything important or pertaining to our salvation. One did not say that the Lord was crucified while another said He was not crucified.

Commentary on Mark ↗

Augustine of Hippo

Y This maid is not the same, but another, as Matthew says. Indeed we must also understand, that in this second denial he was addressed by two persons, that is, by the maid whom Matthew and Mark mention, and by another person, of whom Luke takes notice. It goes on: And he denied it again. …

Catena Aurea by Aquinas ↗

Bede

Again, when the maid saw him, she began to say to those standing around: This man is one of them. But he denied it again. The one accusing him now should not be believed to be the same as before. For Matthew says very clearly: When he went out to the gate, another saw him and said to those who were there (Matthew 26), etc. From this denial of Peter, we learn not only that Christ is denied by someone who says He is not Christ, but also by someone who, when He is, denies that he himself is a Christian. …

On the Gospel of Mark ↗