QUESTION: Rebbi Yehudah maintains that "Semuchin" -- the method of deriving Halachos through the proximity of one verse to another -- may be implemented only if there is a compelling reason ("Muchach") to do so (for example, one of the verses being expounded is out of place and belongs somewhere else), or if one of the verses is extra ("Mufneh").
One such incident of Semuchin which Rebbi Yehudah expounds is the verse, "Lo Yikach Ish Es Eshes Aviv, v'Lo Yegaleh Kenaf Aviv" -- "A man shall not take the wife of his father, and he shall not uncover the cloak of his father" (Devarim 23:1). Rebbi Yehudah utilizes Semuchin to derive that the phrase, "Lo Yegaleh Kenaf Aviv," teaches that one may not marry "Anusas Aviv," a woman who was raped by his father. Which verse is extra ("Mufneh") such that Rebbi Yehudah can apply Semuchin? The Gemara says that the words, "Lo Yegaleh Kenaf Aviv," are extra, since the first half of the verse already teaches that a man may not take his father's wife. Since the second half of the verse is extra, it must be teaching -- through Semuchin (proximity to the previous Parshah which discusses the laws of rape) -- the prohibition against marrying "Anusas Aviv."
The Rabanan -- who argue with Rebbi Yehudah and permit a man to marry a woman who was raped by his father -- understand the verse differently. They explain that "Lo Yegaleh Kenaf Aviv" does not teach the prohibition of "Anusas Aviv," but rather it teaches the prohibition against marrying the "Shomeres Yavam" of one's father (a woman who is bound to perform Yibum with one's father but has not yet performed Yibum or Chalitzah).
Why, then, when the Gemara explains the view of Rebbi Yehudah, does it say that the second half of the verse is superfluous? It is clear from the view of the Rabanan that it is not superfluous -- it teaches the prohibition of "Shomeres Yavam" of one's father. Since that part of the verse is not extra, Rebbi Yehudah should not be able to apply Semuchin to derive the prohibition of "Anusas Aviv." (TOSFOS DH Im Ken)
(a) TOSFOS explains that Rebbi Yehudah considers only one word to be extra, and not the entire second half of the verse. If the intention of the verse would have been to teach only the prohibition of "Shomeres Yavam," it would have said, "v'Lo Yegaleh Kenafav" ("[A man shall not take the wife of his father,] and he shall not uncover his cloak"), without the extra word "Aviv" ("of his father"), since the verse already says "Aviv" in the previous clause and it is clear that "Aviv" (his father) is the subject of the verse. Since the word "Aviv" is superfluous, Rebbi Yehudah has license to apply Semuchin.
(b) The RIVAN answers that there is another reason for why Rebbi Yehudah does not accept that the verse refers to "Shomeres Yavam" of one's father. In the case of the "Shomeres Yavam" of one's father, the woman who is bound to do Yibum with his father (whose brother died childless) is already forbidden to him by two other prohibitions: the prohibition against a Yevamah marrying anyone other than her husband's brother ("Isur Yevamah la'Shuk"), and the prohibition of "Eshes Achi Aviv" (the wife of the brother of one's father). If this verse would refer to the "Shomeres Yavam" of one's father, then it would teach a third prohibition for an act which is already forbidden by two other prohibitions. It is illogical to assume that the intent of the verse is to add a third Lo Ta'aseh to one act. (Although the first half of the verse adds a Lo Ta'aseh to the prohibition of "Eshes Aviv," that is only a second, and not a third, Lo Ta'aseh.) Therefore, Rebbi Yehudah understands that the verse refers only to "Eshes Aviv," and, consequently, the second half of the verse is extra and is able to teach, by way of Semuchin, the prohibition of "Anusas Aviv." (This also appears to be the approach of Rashi (DH Lichtevei).)