Doubly accepted event invitation handling

[ChristophWurst]

Steps to reproduce

1. Receive an event invite
2. Accept it
3. Accept it

Expected behavior

This is somehow handled

Actual behavior

#6956 (comment)

Mail app version

2.0.0 beta 4

Mailserver or service

No response

Operating system

No response

PHP engine version

No response

Web server

No response

Database

No response

Additional info

@st3iny I'm transferring the old review comment. See if you can reproduce. If you can't close the ticket.