Merkkijono

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        String data = "ABCDDCBA";//input.nextLine();

        while (data.matches(".*(\\w)\\1+.*")) {
            data = data.replaceFirst("(\\w)\\1+", "");
        }

        //System.out.println(data.matches(".*(\\w)\\1+.*"));

        System.out.println(data);
    }
}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: WRONG ANSWER

Test 5

Verdict: WRONG ANSWER

Test 6

Verdict: WRONG ANSWER

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: WRONG ANSWER