Oh right, they did all turn to dust, huh? Well, that doesn't necessarily mean there's nothing left in the remains. Plants are known to use ashes as a form of fertilizer.
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Or he could, y'know, double the size of the planet too, as well as all the planets and the ozone layer and so on. He's got literal infinite power, he can do whatever he wants.
Alternatively, he could just copy the planet and all the things on it, and then just split the people and animals across them 50/50.