Read data from arduino once only serial communications (jSSC)

I am trying to get the UID of RFID from Arduino to netbeans. I was able to display it to a textfield, but once another rfid was scanned, it is displayed together with the UID from the prev scan. How do I stop Arduino to send data so the only thing displayed in the textfield was the first UID scanned.

Here is Arduino code:

#include <SPI.h>
#include <MFRC522.h>

#define RST_PIN         9          // Configurable, see typical pin layout above
#define SS_PIN          10         // Configurable, see typical pin layout above

MFRC522 mfrc522(SS_PIN, RST_PIN);  // Create MFRC522 instance

void setup() {
    Serial.begin(9600);     // Initialize serial communications with the PC
    while (!Serial);        // Do nothing if no serial port is opened (added for Arduinos based on ATMEGA32U4)
    SPI.begin();            // Init SPI bus
    mfrc522.PCD_Init();     // Init MFRC522

    //mfrc522.PCD_DumpVersionToSerial();    // Show details of PCD - MFRC522 Card Reader details
    //Serial.println(F("Scan PICC to see UID, SAK, type, and data blocks..."));
}

void loop() {
    // Look for new cards
  String code = "";
    if ( ! mfrc522.PICC_IsNewCardPresent()) {
        return;
    }
    // Select one of the cards
    if ( ! mfrc522.PICC_ReadCardSerial()) {
        return;
    }
  for (byte i = 0; i < mfrc522.uid.size; i++) {
      code += String(mfrc522.uid.uidByte[i] < 0x10 ? " 0" :" ");
      code += String(mfrc522.uid.uidByte[i], HEX);
  }
  code.toUpperCase();
  Serial.println(code);
  mfrc522.PICC_HaltA();
}