Prevent errors for exact duplicate usernames

[rtibbles]

Summary

• Username case error #11716 fixed errors caused by case insensitively matching usernames, but a possible error remained if usernames were exact duplicates (this is possible by the same username being created on different devices, but the same facility and then being synced)
• This adds a regression test and fixes this issue
• Also moves querying of the unauthenticated user until after we have failed to login, to avoid an unnecessary premature query.

References

Depends on #11754, as that involved some updates to the authentication logic, and so avoids a merge conflict.

Reviewer guidance

Check that the newly added test makes sense. Check that it passes.

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

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Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b13.dev0_git.2.geee5d3f4.pex
Windows Installer (EXE)	kolibri-0.16.0b13.dev0+git.2.geee5d3f4-unsigned.exe
Debian Package	kolibri_0.16.0b13.dev0+git.2.geee5d3f4-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b13.dev0+git.2.geee5d3f4-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b13.dev0+git.2.geee5d3f4-0.1.0-debug.apk
TAR file	kolibri-0.16.0b13.dev0+git.2.geee5d3f4.tar.gz
WHL file	kolibri-0.16.0b13.dev0+git.2.geee5d3f4-py2.py3-none-any.whl

[marcellamaki]

this update, and the corresponding test, makes sense to me