fix: validation on same-file generation

[tvillaren]

Fixes #219

Haven't added tests as they would need to be at the CLI level.

I've already spent quite some time trying to configure @oclif/test but I think we're missing some configuration on Oclif for the library (@oclif/test doesn't find the command)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.98%. Comparing base (e21ea92) to head (20175f4).
Report is 1 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #221   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files          18       18           
  Lines         922      922           
  Branches      368      368           
=======================================
  Hits          885      885           
  Misses         37       37           

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