Java使用什么来确定密钥是否在Map中是重复的？

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("A", 1);
        map.put("B", 2);
        map.put("C", 3);
        map.put("D", 1);

        Set<String> keySet = new HashSet<>(map.keySet());
        if (keySet.size() < map.size()) {
            System.out.println("Map contains duplicate keys");
        } else {
            System.out.println("Map does not contain duplicate keys");
        }
    }
}