Hi Asif and Alin,
Thank for your replies. @alinclamba: on laptop/PC I don’t use the special WordPress app from WordPress.com to manage my site (I didn’t want a login at WordPress.com; OK, my quirk??) and then you add messages through your browser. Duplicating in the browser is coping all your blocks into a new message. Then it works OK.
In the WordPress-app from WordPress.com (for iPhone/iPad) made by Automattic, as far as I can see, you have a Duplicate menu-item. And there it goes wrong.
As @asifsiam97 says, it’s probably an app bug. And it doesn’t make any difference if I use the original www.ads-software.com-app or the JetPack-app. I didn’t have it before: the release of the WordPress site on the Raspberry Pi is the same. Don’t know when a new release form the WordPress-app was in between may and august this year.
I see there is a post on GitHub on this, I will add my experience.
Thanks!