My solution

import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

public class Main implements Runnable {
    public static void main(final String[] args) {
        new Main().run();
    }

    @Override
    public void run() {
        final Scanner inputScanner = new Scanner(System.in);
        try {
            final Set<Character> readCharacters = new HashSet<Character>();
            for(final char c : inputScanner.nextLine().toCharArray()) {
                if(readCharacters.contains(c)) {
                    System.out.println("Duplicate found");
                    break;
                } else {
                    readCharacters.add(c);
                }
            }
        } finally {
            inputScanner.close();
        }
    }
}