The Torah.org halakha class says,
If a kohen is present in the synagogue, he must be called to receive the first aliyah. After the Kohen, a Levi is called. If a Levi is not present, the same Kohen is called to the Torah again in his place, the gabbi reciting, bimkom Levi - "in place of a Levi."
Why do we use the same kohen and not call another kohen “bimkom Levi”? Is it because of troubling the congregation or because it might cast slurs on the origin of the second kohen or …?