import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

/**
 * Java Program to demonstrate how to find duplicate words in String.
 */
public class DuplicateWordsInString{

    public static void main(String[] args) {
        String test = "This sentence contains two words, one and two";
        Set<String> duplicates = duplicateWords(test);
        System.out.println("input : " + test);
        System.out.println("output : " + duplicates);
    }


    /**
     * Method to find duplicate words in a Sentence or String
     * @param input String 
     * @return set of duplicate words
     */
    public static Set<String> duplicateWords(String input){
        
        if(input == null || input.isEmpty()){
            return Collections.emptySet();
        }
        Set<String> duplicates = new HashSet<>();
        
        String[] words = input.split("\\s+");
        Set<String> set = new HashSet<>();
        
        for(String word : words){
            if(!set.add(word)){
                duplicates.add(word);
            }
        }
        return duplicates;
    }
    
    
}

Output :
input : This sentence contains two words, one and two
output : [two]