Nested styles (aka combo classes) are in no way an adequate replacement for duplicated styles. Combo classes easily become a mess - e.g. making changes is very difficult and confusing - you may forget to apply a change to the main class and apply it to the the combo class instead… If you want to style an element in the same way as an element with a combo class, you’d need to add two classes. And if you have two combo classes to an element, and want to change something, then welcome to hell…
One scenario for using duplicated styles is templates - let’s save we have an element of 400px width in the template, because it best fits the template design that way. But what if the final user wants to change that template element to fit his own, modified design and wants the width 600px? The easiest way to achieve that modification and preserve the original styles (for reference purposes) at the same time, would be the option to duplicate the original class.
(And just think what shambles would set on if you opt to use combo classes in this scenario.)