It was a theme’s malfunctioning. Of course, the simple solution would have been to change the theme. But, such a step inevitably entails widgets’ and Google Adsense issues: I would have reinstalled some widgets and services.

I found the culprit in the source code of my post: <div class=”entry-summary“> With the help of https://wordpress.stackexchange.com/questions/356530/how-to-edit-source-code-of-specific-page-in-wordpress I managed to insert a piece of code to “Additional CSS”: .entry-summary {display:none;} And duplicate paragraphs throughout my site have disappeared!!!