Having already warned them to separate from the congregation of Korach (in Pasuk 21), why did Hashem see fit to warn them again?
Rashi (citing Targum Onkelos) and Targum Yonasan: Before Hashem ordered them to separate from the congregation of sinners, whereas now He ordered them to move away from their vicinity. 1
Seforno: Before Hashem ordered them to leave the vicinity of Korach's congregation; now he added the vicinity of Dasan and Aviram.
Targum Yonasan: Before he warned Moshe and Aharon to leave the vicinity of all the people; now, having accepted Moshe's Tefilah, we was warning all the people to leave the vicinity of Korach and his congregation. 2
Presumably, this is because now, Hashem added Dasan and Aviram (Refer to 16:24:1:3), they needed to move away, in order not to perish together with the sinners' possessions (See Pasuk 32). See also 16:25:3:1.