What I posted was an example of how to use replace() to fix the error - not the fix for your script. Here is one way to fix your script using replace()
Code: Select allfunction dupLayers() {// newDocFromLayer
var desc143 = new ActionDescriptor();
var ref73 = new ActionReference();
ref73.putClass( charIDToTypeID('Dcmn') );
desc143.putReference( charIDToTypeID('null'), ref73 );
desc143.putString( charIDToTypeID('Nm '), activeDocument.activeLayer.name.replace('/','_') );// this will be the new doc name
var ref74 = new ActionReference();
ref74.putEnumerated( charIDToTypeID('Lyr '), charIDToTypeID('Ordn'), charIDToTypeID('Trgt') );
desc143.putReference( charIDToTypeID('Usng'), ref74 );
executeAction( charIDToTypeID('Mk '), desc143, DialogModes.NO );
};