nixos/sshd: detect duplicate config keys

      '';

    assertions = [{ assertion = if cfg.settings.X11Forwarding then cfgc.setXAuthLocation else true;

                    message = "cannot enable X11 forwarding without setting xauth location";}]

                    message = "cannot enable X11 forwarding without setting xauth location";}

                  { assertion = lib.lists.unique (map (x: lib.strings.toLower x) (attrNames cfg.settings))

                      == (map (x: lib.strings.toLower x) (attrNames cfg.settings));

                    message = "Duplicate sshd config key; does your capitalization match the option's?"; } ]

      ++ forEach cfg.listenAddresses ({ addr, ... }: {

        assertion = addr != null;

        message = "addr must be specified in each listenAddresses entry";