fix: ensure unique imports in generated options

[BobbieGoede]

🔗 Linked issue

• RollupError: virtual:#internal/i18n/options.mjs (5:7): Identifier "locale_de_[...]" has already been declared #3422

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Resolves #3422

Adds reused locale file in test fixture to trigger the (previously) broken behavior to prevent future regressions.

📝 Checklist

• I have linked an issue or discussion.
• I have added tests (if possible).
• I have updated the documentation accordingly.