Stuck invite: duplicate auth_events

### Description

Stuck invite, cannot be accepted or declined just keep repeating. No active account in room to cancel invite. Error when trying to accept:

`Failed to join`

`MatrixError: [403] Event $iUgTqEfK_E9r00YQUk7GbX8Ncwqs3ptL2PgassMwpKY has duplicate auth_events for ('m.room.member', '@tekmad:beeper.com'): $ND2OhQgwc1q2kvX57wjg4ZisgVKhDh8pRpzEuwlqnvo and $2i8vas1r2secWen66__b2i1_Eyw5qNNvdQWRybAxMdY (https://matrix.beeper.com/_matrix/client/v3/join/!TXzTbnCqjPqseRFQAu%3Amatrix.org)`

### Steps to reproduce

Open Element, one pending invite.
Do you want to join TheHaunt?
Accept - duplicate auth_events
Decline - Nothing happens, invite still there.

### Homeserver

matrix.org

### Synapse Version

1.138.0rc1

### Installation Method

I don't know

### Database

?

### Workers

I don't know

### Platform

matrix.org

### Configuration

?

### Relevant log output

```shell
MatrixError: [403] Event $4WGGw2fbvIdXgiN69GmXkILAh1WM1zz3wrzIelxZKc4 has duplicate auth_events for ('m.room.member', '@tekmad:beeper.com'): $ND2OhQgwc1q2kvX57wjg4ZisgVKhDh8pRpzEuwlqnvo and $2i8vas1r2secWen66__b2i1_Eyw5qNNvdQWRybAxMdY (https://matrix.beeper.com/_matrix/client/v3/join/!TXzTbnCqjPqseRFQAu%3Amatrix.org)
```

### Anything else that would be useful to know?

Maybe related to https://github.com/element-hq/synapse/issues/18606

No source files or tests are named. Start by reproducing the pending-invite behavior in Element and compare the duplicate-auth_events failure with related issue #18606; done should be demonstrated by an invite that can be accepted or declined without remaining stuck.

Written by the indexing model from the issue text.