Add duplicate check to CI

[oJshua]

• I'm submitting a ...
  [ ] bug report
  [x] feature request
  [ ] question about the decisions made in the repository
  [ ] question about how to use this project

• Summary
  If a duplicate entry makes its way into the token list, this can cause issues for projects using React where keys are no longer unique.

• Other information (e.g. detailed explanation, stack traces, related issues, suggestions how to fix, links for us to have context, eg. StackOverflow, personal fork, etc.)