There doesn't appear to be an easy way to copy or clone a Site created in Enterprise. It would be extremely useful to have this ability through the interface as the only way to do it at the moment is via Python.
For the end user it could be as simple as having a Save As function in the Site Editor (similar to WAB); or when creating a new site select an existing site to use as a template; or a copy button in the Site Manager.
I'm sure this would be useful in AGOL Hubs too, although Hubs already has a clone function.
This functionality was implemented in ArcGIS Enterprise 10.8 - What’s New in ArcGIS Enterprise 10.8