If there were both a Kohen (Cohen) and a Lewi (Levi) in the Synagogue and the Kohen was called up to the Torah for the 1st Qomah ('Aliyah), and thinking that there was no Lewi started the blessing for the second Qomah also, he must not be stopped. This applies when the Kohen has already said G-d's name in the blessing (Barukh Attah . . .). But if he merely said up to the "Rabbanan Barkhu", it is considered as if he hasn't yet started the blessing.
If there is a Kohen but no Lewi in the Synagogue, the same Kohen must go up for the portion of Lewi and not another Kohen instead. This is so that it will not be said that there was something wrong with the first Kohen and that is why a different one had to go up in his place.
Similarly, two Levites (Lewiyim) do not go up one after the other, so that it will not be said that one of them had a blemish. However, if he was called up to the Torah he should not be sent down.
(See Shulhan 'Aruch, 135: 7, 135: 8 & 9. Kaf Hahayim ibid Oth 47, 48 & 54)
See what people are 
