Duplicate event (with same ID) in ICS export

Steps to reproduce

Unfortunately, I do not know how to reproduce it.

It seems I imported the same event .ics twice in Thunderbird a while ago (2017), and it created the same event twice

Expected behavior

The event should be deduplicated, at least in ICS export of Nextcloud calendar

Actual behaviour

The ICS export contains the same event twice, which crashes imports to Radicale with "412 Precondition Failed".

Here is an excerpt from the .ics file exported from Nextcloud:

BEGIN:VEVENT
CREATED:20171012T201200Z
LAST-MODIFIED:20171012T201911Z
DTSTAMP:20171012T201911Z
UID:8ca61ddc10624d4ee8f1a93ee2a0de616c6dbf99@trainline.eu
SUMMARY:redacted
STATUS:CONFIRMED
ORGANIZER;CN=Trainline;SCHEDULE-AGENT=CLIENT:mailto:calendar@trainline.eu
CATEGORIES:TRAVEL
DTSTART:20171221T110400Z
DTEND:20171221T154800Z
DESCRIPTION:redacted\n\n\n
LOCATION:redacted
SEQUENCE:1
TRANSP:TRANSPARENT
X-MOZ-RECEIVED-SEQUENCE:1
X-MOZ-RECEIVED-DTSTAMP:20171012T201802Z
END:VEVENT
BEGIN:VEVENT
CREATED:20171012T201200Z
LAST-MODIFIED:20171012T201830Z
DTSTAMP:20171012T201830Z
UID:8ca61ddc10624d4ee8f1a93ee2a0de616c6dbf99@trainline.eu
SUMMARY:redacted
STATUS:CONFIRMED
ORGANIZER;CN=Trainline;SCHEDULE-AGENT=CLIENT:mailto:calendar@trainline.eu
CATEGORIES:TRAVEL
DTSTART:20171221T110400Z
DTEND:20171221T154800Z
DESCRIPTION:redacted\n\n\n
LOCATION:redacted
SEQUENCE:1
TRANSP:TRANSPARENT
X-MOZ-RECEIVED-SEQUENCE:1
X-MOZ-RECEIVED-DTSTAMP:20171012T201802Z
END:VEVENT

Calendar app version

No response

CalDAV-clients used

Created with Thunderbird, exported from web UI

Browser

Firefox 102.3.0esr

Client operating system

Debian 11

Server operating system

No response

Web server

No response

Database engine version

No response

PHP engine version

No response

Nextcloud version

Don't know, hosted instance at https://framagenda.org/

Updated from an older installed version or fresh install

No response

List of activated apps

No response

Nextcloud configuration

No response

Web server error log

No response

Log file

No response

Browser log

No response

Additional info

No response

No source file, test, or code entry point is identified in the report. Start by tracing the calendar ICS export path and its handling of repeated event UIDs; confirm the behavior with duplicate events and verify that the export no longer contains duplicate event records.