The below link give you the feature so that your first one that it's basing it's position off doesn't flag an error as it doesn't have a index to refer to. The everything after that (Duplications) will - 10 on the y axis and stay linear on the x.
x = thisComp.layer(index+1).transform.position;
y = thisComp.layer(index+1).transform.position - 10;
x = value;
y = value;