the problem is {x}^{4} - 16

I get;

{x}^{4} - {2}^{4} = {(x+2)}^{2} + {(x-2)}^{2}

for zeros of 2 and -2 each of multiplicity of 2.

However the book states zeros of 2 and -2 each of multiplicity 1. Could someone be so kind as to explain to me what I have done wrong?