In this week's parsha (12:5), in a statement to Moshe referring to the Kohanim, Hashem commands, "They shall not make a bald spot on their heads, and they shall not shave an edge of their beards and in their flesh they shall not cut a gash." This is in response to the fact that many years ago it was customary among many nations to make gashes in one's skin and shave one's head as a sign of mourning.
The Seforno asks why it is necessary to mention these rules specifically for Kohanim, since these laws are mentioned elsewhere (Devarim 41:1) with reference to Bnei Yisrael. Since the laws are no different, what is the reason for emphasizing them again?
He answers (following פשט, the straightforward reading of the text) that since there is already an exception for the Kohen (ie., that he may be exposed to the dead bodies of his relatives and become טמא, but he may not be exposed to the corpses of other people) one may think that there also may be exceptions related to his method of mourning. A Kohen might think that although it is normally prohibited, when a relative dies he is allowed to shave off his hair with a razor and make gashes in his skin. Therefore, the pasuk is repeated here specifically for the Kohanim, to emphasize that there are no exceptions to this איסור.
Rashi cites the Gemara in Makkot which answers this question. The Gemara employs the interpretive method of גזירה שוה (when the same term is used in two different locations, information learned from one source can often be applied to the second source). The Gemara points out that the word קרחה is used in both places. Therefore, just as here in Parshat Emor the Torah refers to shaving any part of the head, also פרשת ראה must be speaking about shaving any part of the head. Similarly, just as the Torah in Devarim is referring to mourning a death, so too in Parshat Emor the rule applies in the context of mourning.