I noticed that it is possible to login multiple times under the same user, which I find odd. When resuming and presented with the lock screen, the focus is sometimes accidentally on "New Session" rather than my user. If I hit enter there I am taken to the sddm login screen with my user. If I login from there, I am logged in multiple times to my own account. At first when I did this, I thought my previous session crashed and I was automatically logged in.

How can this happen? I have a kid who bangs away at the keyboard. While he can't login to my account, he can accidentally set the focus to New Session or to the sddm login screen.

Question: Is there any way to prevent multiple logins to the same account?

With the limits.conf

man limits.conf

NAME
limits.conf - configuration file for the pam_limits module

DESCRIPTION
The pam_limits.so module applies ulimit limits, nice priority and number of
simultaneous login sessions limit to user login sessions. This description of the
configuration file syntax applies to the /etc/security/limits.conf file and *.conf
files in the /etc/security/limits.d directory...

Nice. Thanks!

This method no longer works in as per the latest KDE 5.17.3 (It's what I tested, may not work on lower versions as well)

Try this solution -> viewtopic.php?f=305&t=151924&p=425108#p425108