SEN0359 Fingerprint sensor setDuplicationCheck() confusion

Hello I'm working on a fingerprint matching system that is coded using the Arduino IDE interfacing with an ESP8266 microcontroller. I'm using the module's setDuplicaitonCheck() function to turn the module fingerprint replication check on (Check whether the fingerprint has been registered when saving it). However, I'm confused on its function- I've loaded up the basic registration example and I've set the duplication check to on but the module still allows me to register the same finger print over and over. 

So can anyone tell me how this function is supposed to work and how to actually implement a duplication check for a pre registered finger?

This is the registration example I'm using the only thing I've altered is that I set the duplication check to be on within the void setup():

#include <Wire.h>

/*!

 * @file fingerprintRegistration.ino

 * @brief Fingerprint Acquisition and Saving

 * @details This module can be controlled by hardware serial or software serial

 * @n Experiment Phenomenon：auto retrieve unregistered ID, collect fingerprint 3 times.  

 * @n           In collecting, set LED ring to breathing lighting in blue, and then to quick blink in yellow 3 times when completed

 * @n           At last, save the fingerprint in an unregistered ID, the green LED lights up for 1s and turns off.

 * @copyright  Copyright (c) 2010 DFRobot Co.Ltd (http://www.dfrobot.com)

 * @License     The MIT License (MIT)

 * @author [Eddard]([email protected])

 * @version  V1.1

 * @date  2020-03-19

 * @url https://github.com/DFRobot/DFRobot_ID809_I2C

*/

#include <DFRobot_ID809_I2C.h>

#define COLLECT_NUMBER 3  //Fingerprint sampling times, can be set to 2-3

DFRobot_ID809_I2C fingerprint;

//String desc;

void setup(){

  /*Init print serial port */

  Serial.begin(9600);

  Wire.begin(D6,D1);

  /*Take FPSerial as communication port of the module*/

  fingerprint.begin();

  /*Wait for Serial to open*/

  while(!Serial);

  /*Test whether the device can communicate properly with mainboard

    Return true or false

    */

  while(fingerprint.isConnected() == false){

    Serial.println("Communication with device failed, please check connection");

    /*Get error code information*/

    //desc = fingerprint.getErrorDescription();

    //Serial.println(desc);

    delay(1000);

  }

  //Setting the duplication check feature to be on and checking if its on

  fingerprint.setDuplicationCheck(1);

  int checking = fingerprint.getDuplicationCheck();

  Serial.print("Checking status is set to: ");

  Serial.println(checking);

}

uint8_t ID,i,ret;

void loop(){

  /*Get an unregistered ID for saving fingerprint

    Return ID when succeeded

    Return ERR_ID809 if failed

   */

  if((ID = fingerprint.getEmptyID()) == ERR_ID809){

    while(1){

      /*Get error code information*/

      //desc = fingerprint.getErrorDescription();

      //Serial.println(desc);

      delay(1000);

    }

  }

  Serial.print("unresistered ID,ID=");

  Serial.println(ID);

  i = 0;   //Clear sampling times

  /*Fingerprint sampling 3 times*/

  while(i < COLLECT_NUMBER){

    /*Set fingerprint LED ring mode, color, and number of blinks

      Can be set as follows:

      Parameter 1:<LEDMode>

      eBreathing   eFastBlink   eKeepsOn    eNormalClose

      eFadeIn      eFadeOut     eSlowBlink  

      Parameter 2:<LEDColor>

      eLEDGreen  eLEDRed      eLEDYellow   eLEDBlue

      eLEDCyan   eLEDMagenta  eLEDWhite

      Parameter 3:<Number of blinks> 0 represents blinking all the time

      This parameter will only be valid in mode eBreathing, eFastBlink, eSlowBlink

     */

    fingerprint.ctrlLED(/*LEDMode = */fingerprint.eBreathing, /*LEDColor = */fingerprint.eLEDBlue, /*blinkCount = */0);

    Serial.print("The fingerprint sampling of the");

    Serial.print(i+1);

    Serial.println("(th) is being taken");

    Serial.println("Please press down your finger");

    /*Capture fingerprint image, 10s idle timeout

      IF succeeded, return 0, otherwise, return ERR_ID809

     */

    if((fingerprint.collectionFingerprint(/*timeout = */10)) != ERR_ID809){

      /*Set fingerprint LED ring to quick blink in yellow 3 times */

      fingerprint.ctrlLED(/*LEDMode = */fingerprint.eFastBlink, /*LEDColor = */fingerprint.eLEDYellow, /*blinkCount = */3);

      Serial.println("Sampling succeeds");

      i++;   //Sampling times +1

    }else{

      Serial.println("Sampling failed");

      /*Get error code information*/

      //desc = fingerprint.getErrorDescription();

      //Serial.println(desc);

    }

    Serial.println("Please release your finger");

    /*Wait for finger to release

      Return 1 when finger is detected, otherwise return 0

     */

    while(fingerprint.detectFinger());

  }

  /*Save fingerprint in an unregistered ID */

  if(fingerprint.storeFingerprint(/*Empty ID = */ID) != ERR_ID809){

    Serial.print("Saving succeed,ID=");

    Serial.println(ID);

    Serial.println("-----------------------------");

    /*Set fingerprint LED ring to always ON in green */

    fingerprint.ctrlLED(/*LEDMode = */fingerprint.eKeepsOn, /*LEDColor = */fingerprint.eLEDGreen, /*blinkCount = */0);

    delay(1000);

    /*Turn off fingerprint LED ring */

    fingerprint.ctrlLED(/*LEDMode = */fingerprint.eNormalClose, /*LEDColor = */fingerprint.eLEDBlue, /*blinkCount = */0);

    delay(1000);

  }else{

    Serial.println("Saving failed");

    /*Get error code information*/

    //desc = fingerprint.getErrorDescription();

    //Serial.println(desc);

  }

}

This is the output from the serial monitor, it shows me being constantly registering a finger this is the same finger each time:



The Github for the sensor is here which has the same function with the description: 

https://github.com/DFRobot/DFRobot_ID809_I2C#summary