The Torah is read in the synagogue every Monday and Thursday, and three people are called to the Torah for Aliyot – a Kohen, a Levi and Yisrael. It occasionally happens that a number of Yisraelim are celebrating Semahot on the same morning and would like to receive Aliyot. It could happen, for example, that there is a Bar Misva and a Berit Mila on the same morning, and Aliyot need to be given to the Bar Misva boy, his father, and the father of the newborn baby, but only one Aliya is available for Yisraelim. This problem also arises on occasions when Aliyot are auctioned off, and several Yisraelim purchase Aliyot for large sums to be donated to the synagogue.
In such a case, it is customary to ask the Kohen to leave the synagogue at the beginning of the reading. Once he leaves, all three Aliyot may be given to Yisraelim. If the Kohen does not want to leave the synagogue, or if there are many Kohanim, then the person calling the Aliyot can announce, "Af Al Pi She’yesh Kan Kohen Ya’amod Yisrael Bi’mkom Kohen" – "Even though there is a Kohen, let a Yisrael come in place of a Kohen." This may be done when necessary to provide Aliyot to multiple Yisraelim, but it should not be done on a permanent basis.
(Based on Hacham David Yosef’s Halacha Berura, vol. 7, p. 99)
Summary: If several Aliyot are needed for Yisraelim on a Monday or Thursday, the Kohen should either be asked to leave the synagogue so a Yisrael can receive the first Aliya, or the person calling the Aliyot can call a Yisrael and announce, "Af Al Pi She’yesh Kan Kohen Ya’amod Yisrael Bi’mkom Kohen." However, this should be done only when necessary, and not on a regular basis.