If you mean running NixOS (the distro), rather than just Nix (the package manager), then there’s an option for this `nix.gc.automatic`. You may also want to look at `nix.optimise`. When using just Nix however, I don’t know if they bundle the timers.
`nix.optimise` is just a nix conf option. Meaning anytime nix creates a store path, it will automatically dedup files by hardlinking them to a `/nix/store/.links/` path. There's no timer involved with `nix.optimise`. You can manually force existing paths to be "optimised" by doing `nix-store --optimise`
`nix.optimise.automatic` is an option which creates a systemd timer. The existence of an option that creates a timer suggests that it is not automatically deduping the files (or the option is outdated).