Caching problem in Thunderbird

[ghost]

I noticed that when subscribing to Daylight Calendar in Thunderbird, offline support must be disabled in the calendar properties (see: https://support.mozilla.org/en-US/kb/changing-calendar-preferences).
Otherwise, a new event is created every time the calendar is synchronized, resulting in a very cluttered calendar view.
This doesn't happen with other calendars I subscribed to even with offline support enabled for them.
So I assume that this is a server-side problem with Daylight Calendar?