Hello @Gregor and welcome to the forum!
I don’t think Duplicati can do that directly, as it won’t delete files on restores. Such an option would seem to have some tension with Duplicati’s practice of restoring selected source files. One certainly wouldn’t want everything else on the drive deleted, but maybe everything else in a specific folder that’s specified in the UI.
Rather than delete source folders before restore, it would be safer to restore to a new area, then move the folders to where they belong, after deleting or renaming the curent folders that you’re seeking to mirror into.
Are you asking about a single backup or two? I see you’re asking in 2 Backups available, which to choose?