sources/oauth: fix duplicate authentication

[BeryJu]

Details

fixes #13302

---

Checklist

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)

If applicable

• The documentation has been updated
• The documentation has been formatted (make website)

[BeryJu]

/cherry-pick version-2025.2

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.72%. Comparing base (60521d8) to head (a863260).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13322   +/-   ##
=======================================
  Coverage   92.71%   92.72%           
=======================================
  Files         793      793           
  Lines       40276    40276           
=======================================
+ Hits        37341    37344    +3     
+ Misses       2935     2932    -3     

Flag	Coverage Δ
e2e	48.12% <ø> (+0.05%)	⬆️
integration	24.11% <ø> (ø)
unit	90.43% <ø> (-0.01%)	⬇️

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