We have more than a half dozen projects, some with scores of mockups and many of these are minor variations on a single page view (perhaps with a popup modal or with an item checked exposing a new block of text, for example). In those cases, 95% of the mockups should remain identical since the clones are representing the same page but with slightly more info.
If a change needs to be made to the ‘base’ page that was cloned, that same change has to be made across all clones. It would be very helpful to use the same basic concept of symbols but on a more local scale. So in addition to the ‘clone this mockup’ option, having a ‘clone this mockup as a symbol’ (or clone this mockup with propagation) option where any changes made to the master would propagate to any clones that used the base mockup as a symbol.