SUMMARY
Depending on how you set the checkboxes you either see the event "December Solstice" on Dec 21st not at all, once or twice.

STEPS TO REPRODUCE
1) 

[ ] Astronomical Events
[ ] Holidays

Dec 21: No events

2) 

[ ] Astronomical Events
[x] Holidays

Dec 21: December Solstice


3) 

[ ] Astronomical Events
[x] Holidays

Dec 21: December Solstice

3) 

[x] Astronomical Events
[ ] Holidays

[x] Lunar Phases
[x] Astronomical seasons

Dec 21: No events

4) 

[ ] Astronomical Events (apply) then [x] Astronomical Events (apply)
[ ] Holidays

[x] Lunar Phases
[x] Astronomical seasons

Dec 21: December Solstice

5) 

[x] Astronomical Events
[x] Holidays

[x] Lunar Phases
[x] Astronomical seasons

Dec 21: December Solstice shown twice


SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20211129
KDE Plasma Version: 5.23.3
KDE Frameworks Version: 5.88.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
No further calendars involved. All calendars came shipped built-in.