Mention user using @ in basecamp trix.

Describe the bug or issue here…

##### Steps to Reproduce

1. In base camp trix press @ then user list show like snap https://prnt.sc/1922ix3
2. And select same user multiple time.
3. Now focus on 2nd user it will always focus 1st user https://prnt.sc/19230tn

##### Details
After mention multiple time same user focus on it confuse https://prnt.sc/19230tn

Expect Result: When I select 2nd user at that time focus should be applied to the 2nd user, not 1st user.

* Trix version: 1.3.1
* Browser name and version: Chrome and 91.0.4472.124
* Operating system: Windows

Start by reproducing the repeated-user mention scenario in Trix 1.3.1 using Chrome 91 on Windows, following the listed steps. Trace the mention selection and focus behavior, and consider the issue done when selecting the second identical mention focuses that occurrence rather than the first.

Written by the indexing model from the issue text.