Thunderbird displays calendar entries from various sources, e.g.: - Birthdays from the address book and/or calendars - Holidays from multiple calendars (Spanish holidays, Swiss holidays, German holidays, etc) - Identical Appointments which are same in multiple calendars
This leads to information redundancy and confusion and disabling an entire source is not a practical solution. You should be able to summarize multiple entries, e.g. "Easter Monday" or "birthday of ... " within one day or mark them as "invisible". This should be done within Thunderbird and not uploaded on the server (e.g. Google, etc.).
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