maintainers/scripts/update.nix: deduplicate packages

Lot of my GNOME packages have several legacy aliases,
which causes them to be updated multiple times.

This patch uses lib.unique to remove the duplicates.

maintainers/scripts/update.nix

@@ -9,7 +9,8 @@ let
   pkgs = import ./../../default.nix { };
 
   packagesWith = cond: return: set:
-    pkgs.lib.flatten
+    pkgs.lib.unique
+      (pkgs.lib.flatten
         (pkgs.lib.mapAttrsToList
           (name: pkg:
             let
@@ -25,6 +26,7 @@ let
               else []
           )
           set
+        )
       );
 
   packagesWithUpdateScriptAndMaintainer = maintainer':