We wrote this small plugin to be installed and uninstalled instantly whilst performing a quick check against the database to see if you have any duplicate users which could cause an issue with the Joomla 3.9.16 upgrade.
This plugin will simply check for duplicate users, output a message on screen if duplicate users were found and with a note of the usernames. It will then also email you with information of the duplicated users and the name of the site and of course a link to your site in question.
You can also install this via the YourSites Extension installer which will process it across all sites very very quickly and you will have all responses in your email, or you can click the + on the information about the install on each task.
It's also worth noting, you can use this with Watchful.li and also mysites.guru services. We believe in being inclusive in helping the community wherever we can!
We are happy for any feed back :-)