This is trickpyr's fork of the same repo.

If you click the "installation guide" at the bottom of the page, it will bring you to a document on the original site's repo.
Also, if you go to this site's repo (https://github.com/trickypr/FirefoxCSS-Store.github.io , look at how I changed the domain and moved the username from the subdomain), it will say at the top that this is a fork. The user has a pending merge request, that's the reason they forked it.

If you go to the forks menu of the original repo, you'll find all the repos which you may have seen as duplicates.
Most of them won't be up to date, and they're probably doing it for saving the project, either as a reminder, or in case the repo gets deleted in the future.