* In "nix-env -i", print a warning if there are multiple derivations

  with the same name *and* version number, and pick the first one
  (this means that the order in which channels appear in
  ~/.nix-channels matters).  E.g.:

    $ nix-env ii aterm
    warning: there are multiple derivations named `aterm-2.4.2'; using the first one
    installing `aterm-2.4.2'