Race condition (?) when two guest users join a room simulataneously

### Steps to reproduce

Trafficlight automating two guest users to join a given room at pretty much the same time. One manages to join, the other gets a 403 error.

### Outcome

#### What did you expect?
Both should join
#### What happened instead?
PP: M_FORBIDDEN: MatrixError: [403] You are not invited to this room.

### Operating system

Trafficlight/Linux

### Browser information

Chromium

### URL for webapp

https://github.com/vector-im/element-call/tree/michaelk/data_id_tags

### Will you send logs?

No

Start by reproducing the simultaneous guest join in Trafficlight on the linked webapp revision, using Chromium on Linux. Trace the guest room-join flow and the 403 response; done means both guests can join the same room without receiving the Matrix “not invited” error.

Written by the indexing model from the issue text.