Declining a duplicate community invitation revokes membership

[Zeklandia]

Description

This problem follows from #10773. When a user is invited again to a community they are already a member of and they decline the invitation, they are removed from the community as if the invitation they declined was their original invitation.

Steps to reproduce

• Invite user to a community
• Have user join the community
• Invite the same user to the same community
• Have user decline this second invitation

Describe how what happens differs from what you expected.

I expected the second invitation to be cancelled, removing the user from the "Invited" list.

Logs being sent: no

Version information

• Platform: desktop

For the desktop app:

• OS: Windows
• Version: 1.3.2

[turt2live]

I'm going to close this because fixing matrix-org/synapse#2633 would fix this.