title: Does Nix recognize already installed packages?
body: <p>I want to clarify regarding dependencies installed by Nix. I have a linux installation which has packages D1 and D2 installed. Now I install Nix and then a package through Nix which needs dependencies D1 and D2. Will Nix reinstall D1 and D2 again or will it omit these dependencies since they are already installed on basal linux system?</p>