I might think that even if she had illicit relations with another after she were divorced, she is forbidden to return to him; it is, therefore, written (Ibid. 2) "and he shall write her … (3) and she shall go out, etc.": (Only) one who goes out with a get (from a second husband) is forbidden to return to the first, but not one who had illicit relations with another.
And whence is it derived that if one gives a get to his yevamah (in advance of levirate marriage) she may not return to him? From (Ibid. 4) "Her first husband shall not be able, etc." And whence is it derived that if a woman's husband went abroad, and she was told that he died, and she married another — after which he returned (Whence is it derived) that she needs a get from both? From "Her first husband shall not be able … to return to take her to be to him as a wife."