lib: ensure generators are only defined once

[eclairevoyant]

Addresses #154

Sanity Checking

• My changes fit the guidelines found in CONTRIBUTING guide
• I have tested, and self-reviewed my code
• The unit tests for Hjem pass (nix flake check/nix-build -A checks)
• Style and consistency
  • I formatted all relevant code (nix fmt/nix run -f . formatter)
  • My changes are consistent with the rest of the codebase
  • My commit messages fit the guidelines found in CONTRIBUTING
• If new changes are particularly complex:
  • My code includes comments in particularly complex areas
  • I have included a section in the documentation
• Tested on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin

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