Incorrect number of participants when using contact groups

Steps to reproduce

1. Create two empty contact groups named "Name1x" and "Name2x"
2. Create a contact named "Name 99 name99@example.com" and assign it to both contact groups
3. Create an appointment named "Test 1" with both contact groups "Name1x" and "Name2x" - Save
4. Create an appointment named "Test 2" with the contact group "Name1x" and Save
5. Edit the “Test 2” appointment and add the contact group "Name2x" and save again

Expected behavior

Test 1
Amount of Attendees = 2 (Organisator + Name99)

Test 2
Amount of Attendees = 2 (Organisator + Name99)

Actual behavior

Test 1
Amount of Attendees = 4 (Organisator + Name1x@group + Name2x@group + Name99)

BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
PRODID:-//IDN nextcloud.com//Calendar app 6.5.0-dev.0//EN
BEGIN:VTIMEZONE
TZID:Europe/Berlin
BEGIN:DAYLIGHT
TZNAME:CEST
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
DTSTART:19700329T020000
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
END:DAYLIGHT
BEGIN:STANDARD
TZNAME:CET
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
DTSTART:19701025T030000
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
CREATED:20260616T101615Z
DTSTAMP:20260616T101939Z
LAST-MODIFIED:20260616T101939Z
SEQUENCE:4
UID:7bf93ca0-d727-4a4e-9b68-bd65ed683e64
DTSTART;TZID=Europe/Berlin:20260621T090000
DTEND;TZID=Europe/Berlin:20260621T093000
TRANSP:OPAQUE
STATUS:CONFIRMED
SUMMARY:Test 1 - both at once
DESCRIPTION:Two contact groups with the same contact were added with a sing
 le save
ATTENDEE;CN=Name1x;CUTYPE=GROUP;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT;
 RSVP=TRUE;SCHEDULE-STATUS=5.0:mailto:Name1x@group
ATTENDEE;CN=Name2x;CUTYPE=GROUP;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT;
 RSVP=TRUE;SCHEDULE-STATUS=5.0:mailto:Name2x@group
ATTENDEE;CN=Name99;CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICI
 PANT;RSVP=TRUE;MEMBER="mailto:Name2x@group","mailto:Name1x@group";SCHEDULE
 -STATUS=1.1:mailto:name99@example.com
ORGANIZER;CN=admin:mailto:admin@example.net
END:VEVENT
END:VCALENDAR

Test 2
Amount of Attendees = 5 (Organisator + Name1x@group + Name99 + Name2x@group + Name99)

BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
PRODID:-//IDN nextcloud.com//Calendar app 6.5.0-dev.0//EN
BEGIN:VTIMEZONE
TZID:Europe/Berlin
BEGIN:DAYLIGHT
TZNAME:CEST
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
DTSTART:19700329T020000
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
END:DAYLIGHT
BEGIN:STANDARD
TZNAME:CET
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
DTSTART:19701025T030000
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
CREATED:20260616T101809Z
DTSTAMP:20260616T102012Z
LAST-MODIFIED:20260616T102012Z
SEQUENCE:4
UID:994ea3c7-724b-4fef-b2f0-503343378b27
DTSTART;TZID=Europe/Berlin:20260621T110000
DTEND;TZID=Europe/Berlin:20260621T113000
TRANSP:OPAQUE
STATUS:CONFIRMED
SUMMARY:Test 2 - One at a time
DESCRIPTION:Two contact groups with the same contact were added one after t
 he other
ATTENDEE;CN=Name1x;CUTYPE=GROUP;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT;
 RSVP=TRUE;SCHEDULE-STATUS=5.0:mailto:Name1x@group
ATTENDEE;CN=Name99;CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICI
 PANT;RSVP=TRUE;MEMBER="mailto:Name1x@group";SCHEDULE-STATUS=1.1:mailto:nam
 e99@example.com
ATTENDEE;CN=Name2x;CUTYPE=GROUP;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT;
 RSVP=TRUE;SCHEDULE-STATUS=5.0:mailto:Name2x@group
ATTENDEE;CN=Name99;CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICI
 PANT;RSVP=TRUE;MEMBER="mailto:Name2x@group":mailto:name99@example.com
ORGANIZER;CN=admin:mailto:admin@example.net
END:VEVENT
END:VCALENDAR

Calendar app version

6.5.0-dev

CalDAV-clients used

Browser

Chromium 151

Client operating system

Manjaro Linux

Server operating system

docker-dev

Web server

None

Database engine version

None

PHP engine version

None

Nextcloud version

35.0.0 dev

Updated from an older installed version or fresh install

None

List of activated apps

Enabled:
    - appstore: 2.0.0-dev.0
    - calendar: 6.5.0-dev.0
    - circles: 35.0.0-dev.0
    - cloud_federation_api: 2.0.0-dev.0
    - comments: 2.0.0-dev.0
    - contacts: 8.8.0-dev.0
    - contactsinteraction: 2.0.0-dev.0
    - dashboard: 8.0.0-dev.0
    - dav: 2.0.0-dev.0
    - federatedfilesharing: 2.0.0-dev.0
    - federation: 2.0.0-dev.0
    - files: 3.0.0-dev.0 
    - files_reminders: 2.0.0-dev.0
    - files_sharing: 2.0.0-dev.0
    - files_trashbin: 2.0.0-dev.0
    - files_versions: 2.0.0-dev.0
    - lookup_server_connector: 2.0.0-dev.0
    - oauth2: 2.0.0-dev.0
    - profile: 2.0.0-dev.0
    - provisioning_api: 2.0.0-dev.0
    - settings: 2.0.0-dev.0
    - sharebymail: 2.0.0-dev.0  
    - systemtags: 2.0.0-dev.0
    - theming: 3.0.0-dev.0
    - twofactor_backupcodes: 2.0.0-dev.0
    - updatenotification: 2.0.0-dev.0
    - user_status: 2.0.0-dev.0
    - weather_status: 2.0.0-dev.0
    - webhook_listeners: 2.0.0-dev.0
    - workflowengine: 3.0.0-dev.0

Nextcloud configuration

Web server error log

Log file

Browser log

Additional info

Point 1 - Dedupe Participants

The code for deduplication already exists. See Test 1

The comparison probably fails because the participants in Test 2 have a "mailto" prefix

Ref: https://github.com/nextcloud/calendar/blob/fcb7dda7f92684989c99507234c662f0d01d7522/src/components/Editor/Invitees/InviteesList.vue#L395

Point 2 - No duplicate emails

Because the iTip Broker adds participants to an array before sending out invitations, duplicate emails are prevented (tested on a different instance with NC32 and Calendar 6.4.2)

Ref.: https://github.com/nextcloud/3rdparty/blob/69b1534909f2abd81621d03e3660398743c38ae5/sabre/vobject/lib/ITip/Broker.php#L934

Point 3 - Duplicate Organizer = Attendee
When Thunderbird creates an event, it also adds the organizer as an attendee. So the result of an event with one organizer and one attendee is 3. The calendar counts this multiple times, regardless of the groups. This was noticed during testing and could be also addressed. (tested on a different instance with NC32 and Calendar 6.4.2)

Question: New Issue or same okay?

Point 4 - Group in Event/ICS File
According to RFC, it is sufficient for a participant to be assigned to groups via MEMBER. The group itself does not need to be listed as a participant with "CUTYPE=GROUP". This "group" participant is also displayed by CalDAV clients like Thunderbird.

I can create the PR for this, but before a solution is built, I think we need to decide whether to simply exclude the groups from the count or to omit the groups entirely. The frontend currently needs the groups to render the collapsed section. If the groups are removed, the logic must be adapted to the MEMBER attribute.

Ref.: https://github.com/nextcloud/calendar/blob/fcb7dda7f92684989c99507234c662f0d01d7522/src/components/Editor/Invitees/InviteesList.vue#L186

My recommendation would be:

• The calendar continues to count CUTYPE=GROUP (if the event was created externally with a group via CalDAV client)
• The calendar no longer lists the groups as attendees in the event, but only the attendee as a MEMBER of one or multiple groups
• The frontend builds the expandable groups based on the MEMBER attribute of the attendee and no longer based on the attendees from CUTYPE=GROUP
• The frontend renders an attendee of CUTYPE=GROUP as an attendee

I hope to receive positive and constructive feedback