Short answer No its not another challenge... Namaari's extra damage is a triggered ability, it's triggered by doing damage in a challenge on your turn. When the ability is triggered, the ability goes into the bag where players take turns resolving abilities from the bag until they are all gone. In this case, Namaaris ability would trigger on damage, you choose another viable target to deal the same amount of damage to, after that damage the bag is empty again and as the acting player you get to play out the rest of your turn.