FIX: ensure we only ever enqueue 1 UpdateGravatar job

app/models/user.rb

@@ -753,7 +753,8 @@ class User < ActiveRecord::Base
     avatar = user_avatar || create_user_avatar
 
     if SiteSetting.automatically_download_gravatars? && !avatar.last_gravatar_download_attempt
-      Jobs.enqueue(:update_gravatar, user_id: self.id, avatar_id: avatar.id)
+      Jobs.cancel_scheduled_job(:update_gravatar, user_id: self.id, avatar_id: avatar.id)
+      Jobs.enqueue_in(1.second, :update_gravatar, user_id: self.id, avatar_id: avatar.id)
     end
 
     # mark all the user's quoted posts as "needing a rebake"