First I created in the source installation all the users, and the issues with their own full issue PDF galleys; then I exported all the issues with [Native XML Plugin].
Then I created all the submissions for each of the issues.
In the end I imported the all the users, and then the issues first, and then the submissions.
For each imported submission I found an unpublished duplicated issues (i.e. I found ten times the “Issue 2/2020” and the same thing for every single issue with one submission each).
I successfully delete duplicated unpublished issues; but unsuccessfully find a way to delete duplicated submissions.
Similar thing for user: as soon as I imported the user with [Users XML Plugin] I found all the user roles duplicated with the impossibility to delete them.