There is a well-known Halachic principle called “Kevan She’yasa Mosi,” which means that a person can perform a Misva to fulfill the obligation for somebody else, even though he had already fulfilled that Misva. For example, if a person is asked to blow the Shofar for somebody else on Rosh Hashanah, he may do so even though he had already fulfilled the Misva, reciting the Beracha and blowing 102 sounds. He may even recite the Beracha for that person, despite the fact that he had already fulfilled the Misva. For that matter, if a third person approaches him afterward to request that he blow the Shofar for him, he may recite the Beracha and blow the Shofar. Technically speaking, a person can perform the Misva 600,000 times over, once for every Jew.
Another familiar example of this rule is Kiddush. As we do every Shabbat, one person recites Kiddush on behalf of everyone else at the table, who fulfill their obligation by listening to his recitation. If, after one recited Kiddush, somebody arrives who did not hear Kiddush and does not know how to recite Kiddush, one may recite Kiddush for him. Even though he had already recited Kiddush and fulfilled his obligation, he may perform the Misva again on behalf of the person who had not yet fulfilled the Misva.
The rationale underlying this Halacha is that as long as there is a Jew who has yet to fulfill his obligation, other Jews have still not completely satisfied theirs. And thus even after one has fulfilled a Misva, his fulfillment is not complete, and therefore he is able to fulfill the obligation for others, as well.
An exception to this rule is the Beracha of “Ha’mosi” over bread on Shabbat. As we know, it is a Misva to eat bread on Shabbat, and it is customary for one person to recite the Beracha of “Ha’mosi” over two loaves on behalf of everyone at the table. But if somebody walks in after “Ha’mosi,” and he needs somebody to recite the Beracha for him, one who had already fulfilled the Misva of eating bread may not recite “Ha’mosi” on that person’s behalf. This is the ruling of Maran, in the Shulhan Aruch. He writes that one is allowed to recite “Ha’mosi” on behalf of others on Shabbat only if he is also eating bread at that time. Once he had already recited the Beracha, he may not recite the Beracha on somebody else’s behalf to fulfill the Misva for him, just as in general one may not recite a Beracha over a food or beverage for somebody else unless he is also eating or drinking and requires that same Beracha. Just as somebody cannot recite the Beracha of “She’hakol,” for example, for somebody else unless he is also drinking, one may not recite “Ha’mosi” on Shabbat for somebody else unless he also is beginning his meal and needs to recite this Beracha.
We find two different explanations among the commentators for why “Ha’mosi” on Shabbat differs from other Misvot. The Taz (Rabbi David Halevi Segal, 1586-1667) and Mateh Yehuda (Rabbi Yehuda Ayash, 1688-1760) explain that when one recites “Ha’mosi” over two loaves on Shabbat, it is not evident that this is being done for a Misva. During the week, too, people eat bread and first recite “Ha’mosi.” The rule that allows one to perform a Misva on another person’s behalf after he had already fulfilled the Misva applies only if it is clearly noticeable that the act is being performed as a Misva. We therefore cannot apply this concept to “Ha’mosi” on Shabbat.
The Magen Abraham (Rabbi Abraham Gombiner, 1637-1682) gives a different explanation (cited by the Mishna Berura 167:94). He writes that there is no Misva per se to eat bread on Shabbat. The obligation of Shabbat is to enjoy oneself, and since people generally enjoy eating bread, this is how we fulfill the Misva. However, one who enjoys fasting more than eating fulfills this Misva by fasting, as it gives him more enjoyment, and thus in this case bread is not necessary to fulfill the Misva. Therefore, since bread is not technically obligatory on Shabbat, we do not apply to it the rules that apply to Misvot. The Beracha of “Ha’mosi” on Shabbat is thus no different than any other Beracha over food, which one may recite for somebody else only if he himself requires that Beracha.
Summary: Generally speaking, a person can perform a Misva for somebody else’s behalf even after he had fulfilled the Misva, and thus one who already heard the Shofar on Rosh Hashana can still blow for others, and one who already recited Kiddush on Shabbat can recite it again for others. This is not the case regarding the Beracha of “Ha’mosi” on Shabbat, which one can recite for others only if he himself needs to recite the Beracha over bread.