fix(emails): make sure group notification emails are only sent once per email

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/web/email/group.ex

@@ -26,6 +26,7 @@ defmodule Mobilizon.Web.Email.Group do
         users ++ [Users.get_user_with_activity_settings!(actor.user_id)]
       end
     end)
+    |> Enum.uniq_by(& &1.email)
     |> Enum.each(&notify_follower(event, group, &1))
   end