Adding a user to a repeated event causes duplicate event to be created

Issue summary

1. Editing a repeated event, and adding a user into it causes the event to duplicate itself on that day.

Tested on

1. Confluence 6.15.2 and 6.15.4
2. Team Calendar 6.0.27 and 6.0.32

Steps to replicate

1. Create a weekly repeated event, from Monday to Friday for example, with a single user.
2. Now for the created events, edit any one of it, add another user in into the "Who" section.
3. Save the event, and there will be two events on that day.
   • One will be the older event before the second user was added.
   • And the other will be with the new user added as well.

Current behavior

1. Adding a user into a repeated event causes duplicate entry.

Note: it has also been observed during multiple edits of existing events, that other events are removed from the Calendar.

Expected behavior

1. Adding a user into a repeated event should not cause a duplicate entry, and the existing event should be updated accordingly.

Workaround

1. Delete the older event after.