Trouble in getting RFID tag value from serial port of computer

Question

I am developing a RFID attendance counting system for my classroom. I have interfaced MFRC-522 RFID module with Arduino Uno. The Arduino Uno is connected to a PC via serial port. A C# program reads the serial port to get the RFID tag number and performs matching.

Now, the C# program can successfully read the whole 13-digit number of the card only when it is the first scan. But in the next scans for the same card, the 13-digit number breaks into several parts. The console output of two consecutive tests are shown below:

Press any key to continue...

Data Received: 160975869190 Matched

Data Received: 1 Not Matched

Data Received: 6097 Not Matched

Data Received: 5869 Not Matched

Data Received: 190 Not Matched

I can not figure out why the card number can be read at the first scan but the number breaks into several parts in the next scans? The both Arduino and C# codes are included below:

Arduino Code:

 void setup()
{
  SPI.begin();
  rfid.init();
  Serial.begin(9600);
}

void loop()
{
   if(rfid.isCard())
   {
    RoutineRFID();
   }
}

void RoutineRFID()
{
  if(rfid.readCardSerial())
  {
     Serial.print(rfid.serNum[0]);
     Serial.print(rfid.serNum[1]);
     Serial.print(rfid.serNum[2]);
     Serial.print(rfid.serNum[3]);
     Serial.print(rfid.serNum[4]);
     Serial.flush();
  }

  while(!Serial.available())
  {
  }

  String studentName = Serial.readString();
  lcd.print(studentName);
  delay(3000);
}

C# Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO.Ports;
namespace Test_Serial
{
    class Program
    {

        static void Main(string[] args)
        {
            try{
            SerialPort myPort = new SerialPort();
            myPort.BaudRate = 9600;
            myPort.PortName = "COM6";
            myPort.Open();

            myPort.DataReceived += new SerialDataReceivedEventHandler(DataReceivedHandler);
            Console.WriteLine("Press any key to continue...");
            Console.WriteLine();
            Console.ReadKey();
            myPort.Close();
        }

            catch (Exception ex)
            {
                Console.WriteLine("Com port Disconnected!");
            }
        }

        private static void DataReceivedHandler(
                        object sender,
                        SerialDataReceivedEventArgs e)
        {
            SerialPort sp = (SerialPort)sender;
            string indata = sp.ReadExisting();
            Console.WriteLine("Data Received:");
            Console.WriteLine(indata);

            if (String.Equals(indata, "160975869190"))
            {
                Console.WriteLine("Matched");
                sp.WriteLine("Matched");

            }
            else
            {
                Console.WriteLine("Not Matched");
                sp.WriteLine("Not Matched");
            }

            Console.WriteLine();

        }


    }
}