At the beginning of the last Perek in Sefer Shemos, Hashem instructs Moshe to actually assemble the Mishkan and to begin the ritual service therein (שמות מ':א-'ט"ו). Later (שם פסוקים כ"ו-כ"ז), we are told "וישם את מזבח הזהב באהל מועד לפני הפרכת. ויקטר עליו קטרת סמים כאשר צוה ה' את משה," "and Moshe placed the golden altar in the Ohel Moed in front of the Paroches, and he offered upon it the incense as Hashem had commanded him." Although the Ketores, the incense offering, was but one of many features of the ritual service in the Mishkan and then in the Beis HaMikdash, it subsequently developed its own character and contained provisions shared by no other activity.
The Mishnah at the beginning of the second Perek of Yoma (דף כ"ב.) describes some details as to how the Kohanim carried out their duties within the framework of the Beis HaMikdash. Some background information will be helpful to understand this properly. All Kohanim were originally divided into four Mishmaros, four groups, later expanded to twenty four by Shmuel HaNavi and his Beis Din. Each group was assigned a total of two weeks a year of service in the Mikdash. Initially, anyone from a given week's designated group was permitted to engage in a particular service. Service in the Mikdash often was accompanied by personal reward, for the Kohein that would offer a sacrifice, for example, would be entitled to the part of the animal designated and reserved for Kohanim. The Mishnah (שם) relates that regarding a particular service of the Mizbeiach, whichever Kohein would reach the Mizbeiach first, and beat his competition by four Amos (approximately 6-8 feet, depending on the actual size of an Amoh), would be rewarded with that service. If there was a tie, the two Kohanim would "choose it out" in the identical manner that many people remember "choosing it out" when playing punchball in order to determine who would be up first or last. On one occasion, the Mishnah (שם), documents know that there was a tie and one Kohein pushed his fellow Kohein off the Mizbeiach's ramp and he broke his leg. The Chachomim became alarmed because of the possible danger with this method, and thus replaced it by instituting a פייס, a lottery system. Within each week's group, a lottery was held to determine which Kohein would be assigned to which specific Avodah. If one was lucky enough to win the lottery for a particular activity, his name could nonetheless be put back in the hat the next time his Mishmar was on duty, and if Hashem willed it, his name might be selected a second time.
One exception to this rule, however, did exist. The Mishnah in Yoma (דף כ"ו.) states that when it came to the third of four daily lotteries, namely, that which determines who would offer the Ketores, it was announced "חדשים לקטרת באו," meaning that only those who have never won the Ketores lottery shall be eligible. No individual was ever privileged to offer the Ketores twice. If a Kohein's name came up once, it was then forever removed from this lottery. Why was this the case? Because offering the Ketores carries wealth as its reward, and, as Rashi (שם בד"ה מפני) notes, they did not allow a person to be blessed a second time with wealth, because Hashem wants to "spread the wealth." This idea is predicated upon two Pesukim in Parshas VeZos HaBeracha appearing near one another, one which states that Hashem will bless His "army," and the previous one which contains a description of those who offer Ketores (דברים ל"ג:י'-י"א).
This practice observed in the Mikdash has practical applications as well even today. The Mechaber writes in the Shulchan Aruch ( י"איורה דעה סימן רס"ה סעיף) concerning the Halachos of a Bris Milah that it is our custom to set aside a chair at every Bris and designate it as the chair of Eliyahu (כסא של אליהו). The Ramo (שם) then adds that it is customary to seek the opportunity to become the Sandak, meaning he who holds the child during the Bris. He then notes that the Sandak in fact takes precedence over even the Mohel, in that he is to be honored with an Aliyah to the Torah before the Mohel, for every Sandak is considered to be like one who is offering the Ketores. For this reason, the Ramo concludes, it is customary not to honor one person with סנדקאות, that is, the opportunity to serve as a Sandak, for two children, just as one could not offer the Ketores two times. The Shach (שם ס"ק כ"ב) interprets this to mean that the same person shall not serve as a Sandak for two children within the same family, that is, for two siblings. However, for a child of a second family, it would appear that one may accept the honor again. The Vilna Gaon (ביאור הגר"א שם ס"ק מ"ו) states that the comment of the Ramo (שם) does not make much sense, for if this comparison to the Ketores is correct, one should never be a Sandak twice, even for children of different families. Why limit the restriction to only children within the same family? Furthermore, the Vilna Gaon (שם) adds, we have never seen a Sandak get rich as a result of his having served as a Sandak. He therefore offers an alternate explanation for this practice of not being a Sandak for two children of the same family, citing a statement in the צוואה (will) of Rabbeinu YeHudah HaChassid which indicates simply that we should not allow one individual to serve as a Sandak for two children within the same family. The Shaarei Teshuvah elsewhere (אורח חיים סימן תקנ"א ס"ק ג') raises another objection to the comparison between the Sandak and the Kohein offering the Ketores. In the Mikdash, it was the Kohein who actually brought the Ketores and did the actual Mitzvah who was then blessed with wealth. The equivalent at a Bris would thus be the Mohel and not the Sandak. The Sandak would be the equivalent of the Mizbeiach, and thus should not be the recipient of wealth for simply holding the child.
This practice of not letting any person repeat as Sandak for children within one family is limited to Ashkenazic families. In Sephardic circles, particularly in the Syrian community, the custom is that the paternal grandfather is honored as the Sandak for the first child, the maternal grandfather for the second child, and the pattern is simply repeated for all subsequent children. Several Acharonim point out that if a distinguished Talmid Chochom is present at the Bris, he should he awarded the סנדקאות above anyone else, and it doesn't seem to matter whether he has served as a Sandak for a child in this family or not.
One last note concerning the relationship between the Ketores and serving as a Sandak should be pointed out. The Ketores was comprised of eleven different spices, one of which was חלבנה, a spice that did not have a particularly pleasant aroma. The Midrash explains that the reason it was included in the mix was to symbolize that Am Yisrael as a unit includes all members of the Jewish community, even those that may not as yet be totally committed to our value system. Placing them next to individuals who are שומרי תורה ומצות, observers of Torah and the Mitzvos, will have a positive impact upon them. The Sandak too, whether serving as a Sandak for the child of an observant family or for the child of a family yet to be observant, must now help shoulder the responsibility to convince every child of the idea that he must become an active member of Klal Yisrael. Perhaps this is the reason why the Raavad, cited by the Tur (יו"ד שם), states that there is a wide-spread custom that the Sandak is the one who recites the Beracha of להכניסו בבריתו של אברהם" אבינו," signifying that the child is entering into the Bris of Avraham Avinu, in order to alert him to this awesome responsibility.