Issue description

I want to have gnome3 and pantheon desktop managers in the same NixOS system, so that the user can choose the desired one at login time. So I have on configuration.nix:

  services.xserver.desktopManager.pantheon.enable = true;
  services.xserver.desktopManager.gnome3.enable = true;

But nixos-rebuild gives the following error:

error: The unique option `networking.networkmanager.basePackages.modemmanager' is defined multiple times, in:
 - /nix/var/nix/profiles/per-user/root/channels/nixos/nixos/modules/services/x11/desktop-managers/pantheon.nix
 - /nix/var/nix/profiles/per-user/root/channels/nixos/nixos/modules/services/x11/desktop-managers/gnome3.nix.
(use '--show-trace' to show detailed location information)

cc @worldofpeace

Technical details

• system: "x86_64-linux"
• host os: Linux 5.1.16, NixOS, 19.09pre184855.beff2f8d75e (Loris)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.2.2
• channels(root): "nixos-19.09pre184855.beff2f8d75e"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos