I worked out this output of a sequence:
$Y(z) = \frac{(z-1)(z+\frac{3}{2})}{z^2 (z-\frac{j}{2})(z+\frac{j}{2})(z-\frac{1}{2})}$
My problem is the $z^2$. $z^2 = z * z = (z - 0)(z-0)$
So I think that are 2 poles in the center.
But does that have a special meaning or so ?