Is a blanket that was previously najis considered pure after being washed in an automatic washing machine, or must it be washed again to ascertain its purity, or does the principle of "impurity of something is not decreed merely by doubt" apply?

Washing the blanket in the aforementioned washing machine is sufficient to remove the impurity, and there is no need to wash it again as long as the impurity has been removed. Merely the flow of water over the impure spot is sufficient if the impurity is a legal (hukmi) one.