def key_exists?
      old_message = !@store.msetnx(msg_id, :status =>"incomplete", :expires => @expires_at, :timeout => now + timeout)
      if old_message
        logger.debug "Beetle: received duplicate message: #{msg_id} on queue: #{@queue}"
      end
      old_message
    end
def key_exists?
      old_message = !@store.msetnx(msg_id, :status =>"incomplete", :expires => @expires_at, :timeout => now + timeout)
      if old_message
        logger.debug "Beetle: received duplicate message: #{msg_id} on queue: #{@queue}"
      end
      old_message
    end
have we already seen this message? if not, set the status to "incomplete" and store
 the message exipration timestamp in the deduplication store.