When attempting to select a stacked element, it is very difficult and sometimes impossible to get to the duplicate icon. Mousing over the elements to click the duplicate icon, the builder activates up the elements above, below and behind instead, making the one you're trying to get to impossible. I've had this problem with rows and columns with no margins, too.

Adding duplicate action - like the delete, hide, and move actions - in the sidebar would be an easy way to resolve this.