Since a few days/weeks when duplicating a page all of the element get merged in a huge Text editor block, destroying all layout and making it impossible to use existing pages as base for new layout.
I tried both the “Duplicate page” button in the Pages section in the dashboard and the “Duplicate layout” in the Tools menu while in the Beaver Builder
Weve seen issues with that setting enabled… but last report the user was using jetpack
If you are happy to do some testing, leave it enabled and disable each other plugin in turn to see if you can find the combination that causes the issue.