Glitches horribly – duplicate events show up, repeat doesn't work

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Describe the bug
I tried to add my university schedule to the Elementary Calendar. I only added events by hand, I didn't import anything. After adding a few, the calendar started glitching terribly, making it impossible to manage my events.

[See video.](https://imgur.com/gallery/CrlWaCR)

[Download my exported events.](https://gist.github.com/m93a/80442be791b59249f4d83e8dc0c224fe/raw/0f80bd697b0bd3fd5fa14ccbd743f938931888a2/calendar.ics.zip)

### To Reproduce
Steps to reproduce the behavior:
1. Open calendar
2. Add an event on Monday
3. Turn on weekly repeat
4. See that the event does *not* repeat
5. Try to edit the event
6. A duplicate event appears
7. What.

### Expected behavior
I expected the Calendar app to work properly.

### Screenshots or screen recordings

### Logs
The only error log I saw was this one:
```
** (io.elementary.calendar:3636): CRITICAL **: 11:28:29.710: Store.vala:379: Error from source 'Narozeniny a v?ro??': Cannot invoke method; proxy is for a well-known name without an owner and proxy was constructed with the G_DBUS_PROXY_FLAGS_DO_NOT_AUTO_START flag
```
But this is probably unrelated. I think it was caused by me starting the Calendar with a different locale. ”Narozeniny a výročí” is Czech for “Birthdays and aniversaries”. The glitching appeared even **before** I switched locale and this error log stopped appearing once I restarted the app with the new locale.

## Platform Information

- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version

---
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Start by reproducing the reported sequence in the Calendar app: add a Monday event, enable weekly repeat, then edit it while observing whether repetition fails and duplicates appear. Done means weekly repeats work and editing the event no longer creates duplicate events; the payload names no source files or tests, so the relevant event-handling path must first be located.

Written by the indexing model from the issue text.