Fix(2321): Don't create duplicate CSRs

[ikoenigsknecht]

Avoid creating duplicate CSRs, specifically when creating a user with a duplicated username.

Fixes: #2321

Pull Request Checklist

• [x] I have linked this PR to a related GitHub issue.
• [x] I have added a description of the change (and Github issue number, if any) to the root CHANGELOG.md.

(Optional) Mobile checklist

Please ensure you completed the following checks if you did any changes to the mobile package:

• [ ] I have run e2e tests for mobile
• [ ] I have updated base screenshots for visual regression tests