FIX: mutex to avoid race condition with double notify

app/services/post_alerter.rb

@@ -1,3 +1,5 @@
+require_dependency 'distributed_mutex'
+
 class PostAlerter
 
   def self.post_created(post)
@@ -222,6 +224,7 @@ class PostAlerter
 
     notification_type = Notification.types[:group_message_summary]
 
+    DistributedMutex.synchronize("group_message_notify_#{user.id}") do
       Notification.where(notification_type: notification_type, user_id: user.id).each do |n|
         n.destroy if n.data_hash[:group_id] == stat[:group_id]
       end
@@ -236,6 +239,7 @@ class PostAlerter
           username: user.username_lower
         }.to_json
       )
+    end
 
     # TODO decide if it makes sense to also publish a desktop notification
   end