Wich SOGo server version are you using?
Can you show us the raw event?

SOGo 4.0.2
Raw content? for example dump from database this particular event ?

No, just in the SOGo web interface select the event in your calendar view.
An overlay with details will pop up.
Click on the 3 dots in the right top corner.
Then click on "View Raw Source".
It will display the events source code in iCalendar notation.
Copy that and provide it to us.

Someone with Microsoft Entourage has created/modified this recurring event.
It has created a lot of exceptions, which are placed at the same dates and times, at which the original event takes place.
This is valid.
I do not see, what SOGo can do here.
And I don't know why iPhone is displaying this as multiple events at the same date and time.

PS:
AFAIK Microsoft Entourage is discontinued from Microsoft since 2011 ;-)

I can confirm that issue. With iOS 11 it was possible to deactivate the account wihtin the settings menu and the choose to delete the local calendar. After activating the account, all events are correctly synchronized.
With iOS 12 onward this is not possible anymore, one has to delete the whole calDAV-account and re-add it, then all events are clean and no duplication occurs. But only a limited time.
My guess is, that reorganizations of series are misinterpreted by iOS. Will report

Multiple people are also experiencing this (or similar) issue with SOGo in mailcow-dockerized: see here https://github.com/mailcow/mailcow-dockerized/issues/3019

I think it happens when syncing multiple devices, eg. Create a task with reminder on device 1, then device 2 will sync. After reminder occurs device 2 will keep creating a new one every minute. At least, that's what happened last time which must be a SOGo issue.

I eventually had about 104505 tasks in the system having SOGo slowing down to a crawl. Deleting duplicates from the SOGo database fixed my issue.

After which later it happened again, as long as you have multiple devices and an event/task with reminder.

I can confirm this issue.

Not only on mailcow but also on a non-mailcow SOGo installation. :)

I have the following observations:

1. When adding a reminder on my iPhone by just clicking New reminder and entering the text it gets saved once.
2. When adding a reminder on my iPhone by just clicking New reminder and then clicking the (i) and enter a title and then clicking done, it get's saved twice.
3. When adding a reminder that has an alert to it, it gets duplicated after the reminder has passed.