Issue description

Having both enabled at once in NixOS results in the following error:
error: The unique option 'environment.variables.GDK_PIXBUF_MODULE_FILE' is defined multiple times, in '/home/jaen/Repositories/nixpkgs/nixos/modules/services/x11/desktop-managers/plasma5.nix' and '/home/jaen/Repositories/nixpkgs/nixos/modules/services/x11/desktop-managers/xfce.nix'.

(on the nixos-unstable channel)

Steps to reproduce

  services.xserver.desktopManager.plasma5.enable = true;
  services.xserver.desktopManager.xfce.enable = true;

Technical details

 - system: `"x86_64-linux"`
 - host os: `Linux 4.14.5, NixOS, 18.03.git.8989149 (Impala)`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 1.11.16`
 - channels(root): `"nixos-18.03pre122666.afe9649210c"`
 - channels(jaen): `""`
 - nixpkgs: `/home/jaen/Repositories/nixpkgs`