The Kholiday plugin is active by default. However it's proposed in the plugins to enable. Enabling it makes every event to appear twice. I think it would be better to allow disabling it, but enabling it by default (not hardcoded though)
KHolidays is not enabled by default - I then wiped my config to retest and it seems to work OK. Can you retest please and reopen if there's still a problem.
Created attachment 100128 [details] Config
Created attachment 100129 [details] Calendar See, in the config, by default and currently, holiday is not active, but in the calendar yes
Interesting: Can you paste cat ~/.config/plasma-org.kde.plasma.desktop-appletsrc | grep enabledCalendarPlugins
enabledCalendarPlugins=/home/olivier/Projets/kde/inst/lib64/plugins/plasmacalendarplugins/holidaysevents.so
Ok I tested Plasma beta... I removed the line, and now it's not active by default. Sorry for the noise