Jamieson, Fausset and Brown on Mark 14:69

69. And a maid saw him again--or, "a girl." It might be rendered "the girl"; but this would not necessarily mean the same one as before, but might, and probably does, mean just the female who had charge of the door or gate near which Peter now was. Accordingly, in Mt 26:71, she is expressly called "another [maid]." But in Luke (Lu 22:58) it is a male servant: "And after a little while [from the time of the first denial] another"--that is, as the word signifies, "another male" servant. But there is no real difficulty, as the challenge, probably, after being made by one was reiterated by another. Accordingly, in John (Joh 18:25), it is, "They said therefore unto him, &c.--as if more than one challenged him at once.

and began to say to them that stood by, This is one of them--or, as in Mt 26:71--"This [fellow] was also with Jesus the Nazarene."