Or HaChaim on Deuteronomy 25:5/אור החיים על דברים כ״ה:ה׳

1 לא תהיה אשת המת החוצה לאיש זר, "the wife of the deceased shall not marry a man who is an outsider (stranger), etc." This is a warning to her and to anyone who would cohabit with her that they would be guilty of 39 lashes if they ignored this negative commandment. The Torah also hints here that her brother-in-law, i.e. the potential husband in this levirate marriage, is not obligated to cohabit with her more than a single time. This is in accordance with the opinion of Rabbi Yossi in Shabbat 118 who stated that he engaged in five such cohabitations from each of which a son was born. According to the Jerusalem Talmud Yevamot 1,1 Rabbi Yossi referred to 5 such levirate marriages he was required to endure as he had lost 5 brothers none of whom left behind any children. The additional word ולקחה, "and he shall marry her," is intended to give permission for the surviving brother who has fulfilled the duty of the levirate marriage with his sister-in-law to continue living with this woman in a regular marriage. When the Torah adds the word ויבמה, "and perform the levirate marriage on her," this is intended to counter the argument that everything which has been written by the Torah about this arrangement up until now only gave the brother of the deceasesd the option to perform this cohabitation as distinct from his duty to do so. We could have reasoned that inasmuch as under normal circumstances marrying a sister-in-law is absolutely forbidden, that the Torah relaxed this ruling only in order to give the brother-in-law permission to marry her, but did not obligate him to marry her. This is why the additional word ויבמה is necessary.