Imitation being a form of flattery, the Holland action seems to be the more easily copied, almost every double gunmaker offered a Holland in their catalog. That may also be indicative of the ease of manufacture and repairability.
Beyond blind copiers there were gunmakers who took the Holland a step further. Stefano Zanotti gave it rebounding hammers, improved sears and stronger lump engagement. I have an Italian article on file regarding his improvements if it interests you.
Fratelli Gamba also introduced their own lock improvements. Fabbio Zanotti turned out some fine pinless sidelocks, if you can find them.
