duplicate and movebefore layerSelected

Why do you create a new loop everytime? Of course your variable i will not carry over and the index be all messed up because after the duplication it shifts by +1. Do it all in one.

Mylenium

I did it but it's still not ok

var comp  = app.project.activeItem; 
var layers= comp.selectedLayers;
var numLayers = layers.length;
var myArray=[];
app.beginUndoGroup("Set Marker");
if (numLayers > 0) {
    for (var i = 0; i < numLayers; i++) { 
        myArray.push(layers[i].name); //Get layer name selected
    }
    for (var i = 0; i < myArray.length; i++) { 
        comp.layer(myArray[i]).duplicate(); // Duplicate
    }
    var indexLayer=comp.layer(myArray[0]).index; // Get index
    for (var i = 0; i < myArray.length; i++) { 
        comp.layer(myArray[i]).moveBefore(comp.layer(indexLayer-1));
    }
    }
    app.endUndoGroup();