Guix indeed has a strong commitment to user freedom, which makes it a different project.
Technically, I don't see it as "duplicating" Nix: Guix uses low-level parts of Nix that are awesome, and replaces higher-level layers with a more integrated and unified system. See https://arxiv.org/abs/1305.4584 for the technical rationale or the video on the home page on the concrete impact of those differences. Hacking GuixSD is a very different experience, I think!
(Disclaimer: I maintain Guix and I'm a former NixOS hacker.)