A synagogue was selling Aliyot before the Torah reading on Yom Tob, and two people mistakenly purchased Aliyot for the same person. There was a Rabbi in attendance whom two different people wished to honor with an Aliya, and so one purchased Shelishi (the third Aliya), and the other, not realizing that an Aliya had already been purchased for the Rabbi, purchased Shishi (the sixth Aliya) for the same Rabbi. The question arose as to whether the Rabbi would be allowed to be called up for both Aliyot.
The Rama (Rav Moshe Isserles of Cracow, 1530-1572), in Orah Haim (282), implies that once a person receives an Aliya, he should not receive a second Aliya in the same sequence of Aliyot. In the case described above, then, the Rabbi should choose which Aliya he prefers, and then allow the one who purchased the other Aliya to select somebody else to receive the honor.
However, Hacham Ovadia Yosef ruled that in certain circumstances, when there is a particular need, one may receive two Aliyot in the same Torah reading. One example is the case of a Kohen who is the only Kohen in attendance, and therefore needs to receive the first Aliya, but also observes a Yahrtzeit that day and needs to receive Maftir. In this case, Hacham Ovadia ruled, the Kohen may receive both the first Aliya and Maftir. Likewise, if a Kohen is the only Kohen in attendance at Minha on Yom Kippur, and he wishes to receive Maftir for the honor of "Maftir Yonah," he may receive both the first Aliya and Maftir.
Summary: As a general rule, one may not be called for more than one Aliya in a single Torah reading. This is allowed, however, when a particular need arises, such as in the case of a Kohen who is the only Kohen in the synagogue and observes a Yahrtzeit; the Kohen in this case may receive both the first Aliya and Maftir.