Add mutex around incoming email processing

lib/email/receiver.rb

@@ -39,6 +39,8 @@ module Email
 
     def process!
       return if is_blacklisted?
+      DistributedMutex.synchronize(@message_id) do
+        begin
           @from_email, @from_display_name = parse_from_field(@mail)
           @incoming_email = find_or_create_incoming_email
           process_internal
@@ -46,6 +48,8 @@ module Email
           @incoming_email.update_columns(error: e.to_s) if @incoming_email
           raise
         end
+      end
+    end
 
     def is_blacklisted?
       return false if SiteSetting.ignore_by_title.blank?