Double birthdays event appear in calendar

Expected behaviour

For each day, there should be only one birthday event

Actual behaviour

For every birthday saved in the contacts, two events appear on the day of the occurrence.

Calendar app

Calendar app version: 2.1.3

CalDAV-clients used: MacOS Big Sur Calendar (the problem, nevertheless, is independent from the client)

Client configuration

Browser: Firefox 85.0.2

Operating system: MacOS 11.2.1

Server configuration

Operating system: Linux 5.8.0-41-generic x86_64 (Docker container running in Ubuntu 20.04)

Web server: Nginx

Database: PostgreSQL 12.5 on x86_64-pc-linux-musl, compiled by gcc (Alpine 10.2.1_pre1) 10.2.1 20201203, 64-bit

PHP version: 7.4.14

*Nextcloud Version: 20.0.6

Updated from an older installed version or fresh install: fresh install

List of activated apps:

 Accessibility 1.6.0
Activities for shared file downloads 1.9.0
Activity 2.13.4
AppOrder 0.11.0
Auditing / Logging 1.10.0
Brute-force settings 2.0.1
Camera RAW Previews 0.7.10
Checksum 1.0.1
Collaborative tags 1.10.0
Comments 1.10.0
Contacts 3.4.3
Contacts Interaction 1.1.0
Custom CSS 1.8.0
Deleted files 1.10.1
Draw.io 0.9.8
External sites 3.7.2
External storage support 1.11.1
Extract 1.3.0
Federation 1.10.1
File sharing 1.12.2
Files automated tagging 1.10.1
First run wizard 2.9.0
Full text search 20.0.0
Full text search - Elasticsearch Platform 20.0.0
Full text search - Files 20.0.0
Full text search - Files - Tesseract OCR 20.0.1
HEIC/HEIF Image Converter 1.3.1
Hide sidebars 1.0.7
HSTS Header 0.7.0
Impersonate 1.7.0
Keeweb 0.6.4
Log Reader 2.5.0
Monitoring 1.10.0
Nextcloud announcements 1.9.0
Notes 3.5.1
Notifications 2.8.0
ONLYOFFICE 6.2.0
Password policy 1.10.1
PDF viewer 2.0.1
Photos 1.2.3
Plain text editor 2.14.0
Polls 1.7.5
Preview Generator 3.1.1
Privacy 1.4.0
Quota warning 1.9.1
Ransomware protection 1.8.0
Recommendations 0.8.0
Right click 0.17.0
Scanner 0.2.0
Share by mail 1.10.0
Sharing Path 0.3.0
Support 1.3.0
Tasks 0.13.6
Temporary files lock 20.0.0
Text 3.1.0
Theming 1.11.0
Update notification 1.10.0
Usage survey 1.8.0
User account deletion 1.0.2
User status 1.0.1
Versions 1.13.0
Video converter 1.0.0
Video player 1.9.0
Weather status 1.0.0

Nextcloud configuration:

Dockerized Nextcloud

Logs

Web server error log (e.g. /var/log/apache)

#

Log file (data/nextcloud.log)

#

Browser log

Problem is unrelated to the browser

Start by reproducing the duplicate birthday events with the Contacts 3.4.3 and Calendar 2.1.3 apps, using the reported PostgreSQL and PHP setup if available. Trace how birthdays are turned into calendar events; done means each saved birthday produces exactly one event, independently of the client.