import java.util.*;
import java.io.*;
import java.util.stream.*;
class Ideone
{
	public static void main(String[] args) {
        Map<String, Set<String>> map1 = new HashMap<>();
        Set<String> set1 = new HashSet<>();
        set1.add("AB");
        set1.add("BC");
        map1.put("ABCD", set1);
        Map<String, Set<String>> map2 = new HashMap<>();
        Set<String> set2 = new HashSet<>();
        set2.add("CD");
        set2.add("EF");
        map2.put("ABCD", set2);
        final Map<String, Set<String>> map3 = new HashMap<>(map1);
        map2.forEach((key, val) -> map3.merge(key, val,
                (a, b) -> Stream.concat(a.stream(), b.stream()).collect(Collectors.toSet())));
        System.out.println(map3);
    }
}

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