fix(config): ensure the actually used config file is correct, fix single config file being detected as duplicated, better test coverage

[bearomorphism]

This is an improvement based on #1773

• Improved test coverage of multiple config file warning
• Ensured the consistency of actually used config file, which is not always the first element of the candidates

Closes #1787

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.88%. Comparing base (aa82b98) to head (24e3239).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1784      +/-   ##
==========================================
- Coverage   97.95%   97.88%   -0.08%     
==========================================
  Files          60       60              
  Lines        2646     2648       +2     
==========================================
  Hits         2592     2592              
- Misses         54       56       +2     

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