So ... Let's see the HOW part ...

We are going to use a handy plugin called Prevent Concurrent Logins.

Here it is:

https://wordpress.org/plugins/prevent-concurrent-logins/

Note: you'll see the usual warning message saying that the plugin hasn’t been tested with the latest major releases of Wordpress ... Ignore it ... I've tested the plugin on Wordpress 5.2 and works fine.

As I said, this plugin is ideal for membership sites and web applications. The plugin ...

• deters members and subscribers from sharing their accounts with others
• hardens security by destroying old sessions automatically
• prompts old sessions to log in again if they want to continue

It's a very small plugin (12 KB), but let's see the usual speed tests ...

Here are my results before installing the plugin ...

... and the result after installing and activating the plugin:

No more comments needed ...

Now let's see the magic ...