Multiple mention for the same person

### Steps to reproduce

1. Where are you starting? What can you see?
You can go to any room and click on a person's name appearing on top of their authored message. On clicking multiple times you can see that the same name


### Outcome

#### What did you expect?
Expected only one name when clicking on the same person's name multiple times

#### What happened instead?
On clicking multiple times you can see that the same name is repeated in the message box

### Operating system

Windows 10

### Browser information

Brave
[Version 1.48.158 Chromium: 110.0.5481.77 (Official Build) (64-bit)](https://brave.com/latest/)

### URL for webapp

[site](https://app.element.io/#/room/#element-dev:matrix.org)

### Will you send logs?

No

Reproduce the issue in the linked Element web app by clicking the same person's name repeatedly in a room. Trace the mention insertion behavior in the Element web client; done means repeated clicks leave only one occurrence of that person's name in the message box, with manual verification in Brave on Windows 10.

Written by the indexing model from the issue text.