Rewrite duplicate-checker workflow

[nickspaargaren]

Rewrite Python script to use as the duplicate-checker workflow:

python3 convert.py duplicates

^{Script currently checks for duplicates and prints the duplicates.}

May use with pytest used in: https://github.com/nickspaargaren/no-google/blob/master/scripts/test_duplicates.py

Workflow file: https://github.com/nickspaargaren/no-google/blob/master/.github/workflows/duplicate-checker.yml