TYPO3 Core

Do you know what steps I can follow to reproduce this issue? Can't reproduce on current main

The steps can be reproduced with a bad network connection, or / and with a huge amount of records to copy. Latter one can be easier achieved by choosing pages with many sub-pages.

In my opinion it's not very useful to reproduce it as it's only to show the problem, but a whole new concept has to be implemented.
So in general a task list for the copy-process should be saved somewhere and then the tasks processed step by step, perhaps even with sub-tasks reflecting deeper page hierarchies or many included records.

I think re-rendering of the browser can cause the issue too, as the page with exact the same request is reloaded then.
But like I said: The whole procedure should be changed.