This is what I did:
Duplicate Site in site manager
setting the domain of the new site to a non existing domain name
setting the url path of the source site to something like "/2018"
setting the domain of the target site to the original domain and the url path to "/"
When doing a ?fwreinit=true on the url path "/" the following error is thrown:
Error when adding site tree page. You have not supplied a parent page and there is already a root page for the site. There can only be a single root page per site
Calling the source site with the path "/2018" works fine.
The only way to get the new site running is by setting different domainnames in the site settings.