If a Kohain has a blemish, “he should not come close to offer G-d’s bread” (Vayikra 21:17). Rashi tells us that the term “bread” refers to more than just actual bread; “G-d’s bread” refers to “G-d’s food” (i.e. the offerings), as “every meal (even if not literally bread) is called ‘bread,’ as it says (Daniel 5:1) “Belshatzar the king made a great bread,” which obviously means “made a large meal” (as opposed to baking a loaf of bread large enough to feed “a thousand of his officers”). Four verses later (21:21), when the term “bread” is used again, Rashi again tells us that “all food is called bread.” Why did Rashi tell us this twice, within such a short span?
The most widely given answer (based on the Sifra explaining that the Torah mentioned the term “bread” twice so that we shouldn’t think it only refers to the twice-daily “Tamid” offering) is that the first mention only teaches us that a “meal” is referred to as bread; non-bread food items that aren’t part of a regularly scheduled sit-down meal might not be. Rashi therefore explains the second “bread” as teaching us that “all food” can be referred to as “bread,” not just food served at a “meal.” The “Tamid” offering, which is brought every morning and every afternoon, would be considered a “regular meal,” and we therefore might have thought that a Kohain with a blemish is only forbidden from bringing this kind of offering; the second usage of “bread” therefore teaches us that he cannot bring any offering.
Aside from needing to read more into Rashi than he says (that “meal” refers to the “Tamid” because it is scheduled every day, while “all food” includes offerings not brought every day), if Rashi’s point was that the first usage only teaches us that this Kohain cannot bring a Korbon Tamid, instead of taking the longer route of quoting a proof-text (from the Writings) that “meals” are called “bread” and then relying on us to figure out that “meal=Tamid” and “all food=all offerings,” he could have directly quoted a verse from the Torah (Bamidbar 28:2) that explicitly refers to the Korbon Tamid as “bread.”
There are other issues to be addressed as well. First of all, the word “bread” appears twice in a previous section (Vayikra 21:6 and 21:8), and there Rashi does not explain what “bread” refers to, or how it can refer to offerings. [Some suggest that “bread” in these verses could refer to the “showbread,” or to meal-offerings, which are literally “bread,” so doesn’t need to be explained. However, since it really does refer to figurative “bread,” i.e. the offerings, why didn’t Rashi tell us this earlier?] Additionally, towards the beginning of Sefer Vayikra (3:11) Rashi already told us that “bread” refers to all kinds of food (the verse there is speaking about voluntary “Sh’lamim,” which is as far from “regularly scheduled” as an offering can get); why would he revert back to limiting “bread” to the Korbon Tamid before re-including the other offerings?
How did the word “bread” come to mean “all foods”? The commentators (specifically when discussing its meaning extending from “meals that include bread,” to non-bread food items even when not part of a meal) say it is a “borrowed term,” initially referring to something specific but then evolving to refer to things that can be traced back to the initial meaning. In this case, “bread” literally means actual bread, but since bread is a food staple (see Rashi on Sh’mos 16:8), and the main part of every meal, with everything else served being in addition to — and secondary to — the actual bread (which is why the blessing on bread, “Hamotzie,” covers everything served as part of the meal), the entire meal (and not just the bread itself) was referred to as “bread.” After the common usage of the term “bread” became the entire meal — even the non-bread items served at the meal — the term came to mean all food, even when not served/eaten with actual bread. To sum it up, the evolution of the meaning of the word bread could be described as: actual bread–>meal (where bread is served)–>all food. In the Torah, it refers to what the term has come to mean, i.e. all food, not just those served as part of a meal (as Rashi tells us on Vayikra 3:11).
Aside from Rashi not needing to explain that “bread” in 21:6 and 21:8 means “all food” because we already know this from 3:11, its precise definition in these two verses makes no practical difference, as it doesn’t impact what Kohanim can or cannot do. Rather, it explains why they have additional limitations; “because they offer G-d’s bread.” Whatever “G-d’s bread” means, they offer it, and are therefore “holy.” When it comes to a Kohain with a blemish, on the other hand,” it is “G-d’s bread” that he cannot offer, so we have to know exactly what it is that he can’t offer. And even though the term “bread” evolved to mean “all food,” we need to know whether the Torah is referring to what the term has come to mean, or only to what it used to mean.
Our edition of Rashi (on 21:17) says “every meal is called ‘bread,” but the first edition of Rashi (as well as several other manuscripts, including the edition used by Mizrachi, see Sefer Yosef Halel) says “the entire meal is called ‘bread.” Rather than telling us that all meals are referred to as “bread,” Rashi is telling us that all the foods, even the non-bread items, served at a meal are referred to as “bread,” since bread is served at (and is the main component of) the meal. In other words, whenever non-bread items are served with bread, all of the items being served are referred to as “bread” (the second of the three stages in the development of the term “bread”). When it comes to the offerings brought in the Temple, this would mean any offering that is accompanied by a meal-offering (since the meal-offering itself is literally “bread”). And all we would know for sure is that Kohanim with a blemish cannot bring any offerings that are accompanied by a meal-offering (such as the Korbon Tamid, which might be what the Sifra means). By repeating the term “bread” a few verses later to include additional things this Kohain cannot offer, the Torah is telling us that when it says “bread” it is not referring only to offerings that are accompanied by meal-offerings (as the term once meant), but to “all food,” even when not part of a meal with bread. Or, as applied to offerings in the Temple, even those not accompanied by a meal-offering. As stated in the Sifra, the repetition of the term “bread” expands the definition to teach us that even such offerings cannot be brought by a Kohain with a blemish.
Rashi uses the verse from Daniel (as opposed to the one in Bamidbar), because he is not trying to say that the Korbon Tamid is referred to as “bread,” but that this verse can only teach us that offerings accompanied by meal-offerings, which parallel food served at a meal with bread, cannot be offered by a Kohain with a blemish. The second verse (21:21), on the other hand, which repeats the term “bread,” teaches us that it refers to all food, i.e. even those offerings that are not accompanied by a meal-offering.