Handling duplicate notifications from superfeedr

I have a subscription to an RSS feed via SuperFeedr, and have noticed a lot of duplicate notifications lately. I've double checked my subscriptions via the web application, but I only see 1. I also looked into making sure that I am returning the correct response code. The documentation says that a 200 should be sent as the response, and the retry structure is after 5, 10, and 15 seconds, but in my logs, I see the duplicates being sent milliseconds apart, which leads me to believe that these are not retries.

The very FIRST lines of my doPost() method are:

    response.setStatus(HttpServletResponse.SC_OK);    response.flushBuffer();

The attempt there is to let superfeedr know that I received the message, an ack essentially.

As a short term remedy, I have decided to keep a record in the datastore that represents the 'most recent message'.

I know that normally within the context of a servlet, you wouldn't synchronize on the servlet instance itself, because that would cause a queue of requests, but in this case, Superfeedr is the only service using this servlet, it isn't consumed by normal users, so that is exactly what I want, just wanted to make sure that there wouldn't be any ill side effects of this approach.

I want to allow time for a request to come in from Superfeedr, analyze the message, make sure it's not a duplicate, and if it's not, update the value in the datastore that represents the most recent message, one at a time. Here is what I have:

    synchronized(this) {        //check datastore entry to see if the message coming in is a duplicate        if(messageIsADuplicate(title)){            logger.log(Level.SEVERE, "DUPLICATE MESSAGE RECEIVED");            return;        }        //delete the entry in the datastore that represents the most recent message        if(!mostRecentMessageDeletedFromDatastore()){            logger.log(Level.SEVERE, "UNABLE TO DELETE MOST RECENT MSG");            return;        }        //add an entry to the datastore that represents the most recent message from superfeedr        if(!mostRecentMessageUpdatedInDatastore(title)){            logger.log(Level.SEVERE, "UNABLE TO UPDATE MOST RECENT MSG");            return;        }    }

So, assuming that I want one at a time access to this particular block of code, does this seem appropriate?