Q: My holidays-subscription to iCal shows each event anywhere from 1 to 8 times on the given day. This changes several times a day. How can this be fixed?

I have a subscription to a holiday-calendar in my iCalendar, similar to e.g. US-holiday. Sometimes, the events from this calendar are shown up to 8 times each, but this changes regularly. When I remove the calendar and add it again, it first looks normal, but after a few minutes the events starts being shown several times each. How can I fix this?