Changed repeating calendar entry shows twice

[ToBeHH]

The problem

Assuming you have a calendar with recurring events. Then you modify one instance of the recurring event. The expected behavior is, that the calendar shows only the modified instance of the event.
However, both calendar entries are shown now.

What version of Home Assistant Core has the issue?

core-2025.4.3

What was the last working version of Home Assistant Core?

No response

What type of installation are you running?

Home Assistant OS

Integration causing the issue

Remote Calendar

Link to integration documentation on our website

https://www.home-assistant.io/integrations/remote_calendar

Diagnostics information

This is an example of the original event:

BEGIN:VEVENT
CREATED:20240714T130457Z
DTSTAMP:20240919T054400Z
LAST-MODIFIED:20240919T054400Z
SEQUENCE:3
UID:0defc4f1-d1a5-4e1b-9b5b-b6a84a8657dc
DTSTART;TZID=Europe/Berlin:20240717T193000
DTEND;TZID=Europe/Berlin:20240717T210000
STATUS:CONFIRMED
SUMMARY:Original Summary
RRULE:FREQ=WEEKLY;INTERVAL=2;BYDAY=WE
EXDATE;TZID=Europe/Berlin:20250101T193000
END:VEVENT

This is an example of the modified event:

BEGIN:VEVENT
CREATED:20240919T055809Z
DTSTAMP:20250412T202325Z
LAST-MODIFIED:20250412T202325Z
SEQUENCE:6
UID:0defc4f1-d1a5-4e1b-9b5b-b6a84a8657dc
DTSTART;TZID=Europe/Berlin:20250423T193000
DTEND;TZID=Europe/Berlin:20250423T210000
STATUS:CONFIRMED
SUMMARY:Modified Summary
RECURRENCE-ID;TZID=Europe/Berlin:20250423T193000
LOCATION:Some location
DESCRIPTION:Some Description
END:VEVENT

Note, that the events have the same UID. And I guess the Recurrence-ID is important in this case.

Example YAML snippet

Anything in the logs that might be useful for us?

Additional information

No response

[home-assistant]

Hey there @Thomas55555, mind taking a look at this issue as it has been labeled with an integration (remote_calendar) you are listed as a code owner for? Thanks!

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remote_calendar documentation
remote_calendar source
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[ToBeHH]

Might be related to allenporter/ical#278

[SdeGeata]

Having the same issue. I tried deleting the calendar and reinstalling it, both entries came back.

[Thomas55555]

It's not possible to modify entries in the remote calendar. Maybe you mean another integration.

[Thomas55555]

@home-assistant unassign remote_calendar

[ToBeHH]

@Thomas55555 The remote calendar has this entry. It happens, if you have a repeating event (see example 1), but then change one occurrence of this repeating event series. Then you happen to have 2 calendar entries in iCal (with one referring to another).
So it is not about editing the remote calender. The problem is, that the remote calendar does not handle this type of calendar correctly.

[ToBeHH]

From what I can see, remote_calendar relies on the allenporter/ical library. The library says on its README:

It is not a goal to support everything exhaustively.

And this issue has already been reported to this library (see link above).

At the end of the README, the allanporter/ical lists 2 other libraries - which are quite old (but iCalendar format is quite old as well). Maybe they support the iCalendar format more?

[Thomas55555]

So you modify the event on the server and after home assistant makes an update you have the problem?

[ToBeHH]

Well, yes. So the server in this case is a Nextcloud server. I modify the events there. The Nextcloud calendar shows the event correctly (only once). Home Assistant fetches the data from the Nextcloud server and then the event is shown twice (without doing any further modifications to the event or filtering or anything, just using plain remote_calendar integration and showing the event on the HA internal calendar).

The same Nextcloud calendar is also imported on my Mac (Calendar app) and my Android phone (via DAVx). In both cases, the calendar is show correctly (only once).

[SdeGeata]

Same for me; modification on my calendar event (from any device, on the calendar server), leads to a second event in remote calendar. The old one and the modified one. This behaviour repeats every time the event is modified, clogging the calendar.

[home-assistant]

remote_calendar documentation
remote_calendar source