Or HaChaim on Numbers 16:17/אור החיים על במדבר ט״ז:י״ז

1 חמישים ומאתים מחתות, ואתה ואהרון "two hundred and fifty censers as well as you and Aaron." All these words are superfluous in view of what Moses said in verse 16, i.e. "you and your whole congregation as well as Aaron." If all Moses wanted to tell us was how many people were going to appear with the censers this should have been part of verse 16. The words "you and Aaron" are superfluous whichever way you look at it. Perhaps Moses was clever enough to tell Korach not to become part of the 250 men and their censers but to compete only with Aaron. He may have done so to prevent people from claiming that the only reason Korach's incense had not been accepted was because he had become part of the evil congregation of 250 men, but that in a straight confrontation with Aaron he would have won out. Even though all the people offering incense did so at the same time, this did not prevent them from doing so in two groups, i.e. the 250 men on the one hand and Korach and Aaron on the other.