Issue description

The unique option services.xserver.displayManager.job.execCmd' is defined multiple times in /nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/nixos/modules/services/x11/display-managers/sddm.nix' and `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/nixos/modules/services/x11/display-managers/gdm.nix'.

Steps to reproduce

nixos-rebuild switch --upgrade

Technical details

first try with nixos-18.09

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

• system: "x86_64-linux"
• host os: Linux 4.14.71, NixOS, 18.03.133245.d16a7abceb7 (Impala)
• multi-user?: yes
• sandbox: no
• version: nix-env (Nix) 2.0.4
• channels(root): "nixos-18.09beta739.6cbd1ec4b04"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs