We installed Yoast at the beginning of October, updated the Yoast Plugin on 10th October and discovered the problem on 10th October.
We've tried hard to establish how this could have happened and consider it is Yoast connected.
If we unplug Yoast the problem disappears. If we re-install Yoast the problem re-occurs.
The problem is: If we add text into the main text area of any listing, then that text is added to the text areas of all other listings that have empty main text areas that reside within the same category.
It does not affect listings which already contain text within the main text area.
The problem exists throughout the site and we have uninstalled the contaminated site and re-uploaded a backup version.
We were systematically Yoast SEO-ing the listings on the site at a time when we were experiencing an amount intermittent connectivity from our ISP. We have quite an amount of previous experience with intermittent connectivity but have never experienced such things as multiple data entry before.
The following may be of assistance to you. We have placed a copy of the website in a demo folder with the latest database version and Yoast installed and active.
going to http://businessdirectory.na/category/namibia-architects/
one can see the duplication of data issue
You can login as usual here http://businessdirectory.na/demo/wp-admin