nixos/matrix-synapse: clarify that `extras` are additive

nixos/modules/services/matrix/synapse.nix

···
 
           Extras will automatically be enabled, when the relevant
           configuration sections are present.
+
+          Please note that this option is additive: i.e. when adding a new item
+          to this list, the defaults are still kept. To override the defaults as well,
+          use `lib.mkForce`.
         '';
       };
 

pkgs/servers/matrix-synapse/wrapper.nix

···
 }:
 
 let
-  extraPackages = lib.concatMap (extra: matrix-synapse-unwrapped.optional-dependencies.${extra}) extras;
+  extraPackages = lib.concatMap (extra: matrix-synapse-unwrapped.optional-dependencies.${extra}) (lib.unique extras);
 
   pluginsEnv = matrix-synapse-unwrapped.python.buildEnv.override {
     extraLibs = plugins;