[Script] Duplicate and add same scale value

You can do this really easy with just an expression in the scale.

x = index * 20 + value[1];
[x,x]

You may need to offset the index if the shapes aren't at the top of the layer stack. You can do this manallly, hooking it up to a slider or make a null layer that sits at the top of these layers and use 

-parent.index

If you want to do it via script.

var selectedLayer = app.project.activeItem.selectedLayers[0];
var multiplier = 10;

for (var i = 1; i <= multiplier; i++){
        var dupedLayer = selectedLayer.duplicate();
        amount = dupedLayer.transform.scale.value[0] + 20 * i
        dupedLayer.transform.scale.setValue([amount,amount])
    }

If the shapes have animation on them, then it gets a bit more complicated.