Welcome to Webflow and the Webflow Community.
When you duplicate (copy/paste) an element (in this case a section), the styling (class name) associated with that element is also copied across, therefore when you make a change to the class, all elements with that class will be affected.
To ‘de-couple’ the duplicated element, you will need to ensure it has a different class name. That can be achieved by giving it a completely new name (and therefore loos all styling from the previous class name), or you apply a ‘combo class’ and apply the separate styling.
Hope that helps,