The Torah has already informed us (in the previous Pasuk) that Yosef recognized his brothers. Why does it repeat it here?
Rashi #1: The Torah is pointing out here the difference between him and them; he recognized them because when he left them they already had full beards, but they did not recognize him because his beard had not yet fully grown.
Rashi #2: The Torah is pointing out here that when his brothers fell into his hands, he had mercy on them and treated them as brothers, whereas when he fell into their hands, they did not treat him as a brother.
Ramban: After they told him that they came from Kena'an, any remaining doubts were dispelled and he knew for sure that they were his brothers.
Ramban (citing the Ibn Ezra and Seforno): At first, he merely recognized them as a group, and then individually.
Oznayim la'Torah #1: According to the Midrash
Why did Yosef's brothers not recognize him?
Rashi and Rashbam #1: Refer to 42:8:1:1.
Ramban, Ohr ha'Chayim: Because, unlike Yosef, who anticipated their arrival, it would never have occurred to them that the brother they had sold as a slave to the Yishme'elim was the ruler of Egypt.
Seforno: Refer to 42:7:2:1.
Rashbam #2: Because they now saw him as a royal figure, wearing royal robes. 1
Rashbam #3, They did not recognize him by his voice, because he did not speak to them directly, only via an interpreter.
Malbim: It is because he spoke to them harshly. Also refer to 42:7:2:3.
And a turban that partially covered his face (Ramban).