I'm certain you know the definition of this principle, that something which we could have derived using a Kal v'Chomer will be written by the Torah anyway. The Gufei Halachos (Letter Mem, rule #329) adds that it is possible one could possibly find (or think he had found) a question to ask on the Kal v'Chomer if the extra Pasuk would not have been written. In addition, he notes, it does not always mean that the Torah writes a Pasuk both by topic A and topic B. It is also used in Sanhedrin (41a) as an answer for why there are two Pesukim by topic B. The Torah wanted the second Pasuk to be used in the style of "Im Eino Inyan etc." (if it is not needed to teach us this lesson by topic A, it must be meant for topic B). Even though the Torah could have simply written one Pasuk and not used the "Im Eino Inyan" method, the Gemara says that the Torah will give a second Pasuk anyway.

However, as logic dictates, this is not the type of answer that is normally a first choice answer, as usually we derive something from every Pasuk (see also Gufei Halachos Letter Tzadik, #481). This is especially true according to the opinion that "Shnei Kesuvim ha'Ba'im k'Echad Ein Melamdim." The opinion "Melamdim" is clearly more comfortable with this answer. For more, see Gufei Halachos (and other Sefarim about Klalei Ha'Shas) at length.

All the best,

Yaakov Montrose