i think the test fails because of whitespace changes, so once you get a solution, copy it to a text editor, then paste in only the changes to the comments to make sure you haven't changed anything else, that got the tests to pass for me, even though the output already matched

i guess the test is wrong. the expected output is exact as my generated input, anyway test dont pass.

public class DuplicateWords {

public static void main(String[] args) {

    String regex = "(?i)\\b(\\w+)\\b(\\s+\\1\\b)+";
    Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE);

    Scanner in = new Scanner(System.in);
    int numSentences = Integer.parseInt(in.nextLine());

    while (numSentences-- > 0) {
        String input = in.nextLine();            
        Matcher m = p.matcher(input);
        while (m.find()) {
            input = input.replaceAll(regex, "$1");
        }
        System.out.println(input);
    }
    in.close();
}

}

Do not change code other than three lines

import java.util.Scanner; import java.util.regex.Matcher; import java.util.regex.Pattern;

public class DuplicateWords {

public static void main(String[] args) {

    String regex = "\\b(\\w+)\\b(?:\\s+\\b\\1\\b)+";
    Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE);

    Scanner in = new Scanner(System.in);
    int numSentences = Integer.parseInt(in.nextLine());

    while (numSentences-- > 0) {
        String input = in.nextLine();

        Matcher m = p.matcher(input);

        // Check for subsequences of input that match the compiled pattern
        while (m.find()) {
            input = m.replaceAll("$1");
        }

        // Prints the modified sentence.
        System.out.println(input);
    }

    in.close();
}

}

import java.util.Scanner; import java.util.regex.Matcher; import java.util.regex.Pattern;

public class DuplicateWords {

public static void main(String[] args) {

    String regex = "\\b(\\w+)(\\s+\\1\\b)+";
    Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE);

    Scanner in = new Scanner(System.in);
    int numSentences = Integer.parseInt(in.nextLine());

    while (numSentences-- > 0) {
        String input = in.nextLine();

        Matcher m = p.matcher(input);

        // Check for subsequences of input that match the compiled pattern
        while (m.find()) {
            input = m.replaceAll("$1");
        }

        // Prints the modified sentence.
        System.out.println(input);
    }

    in.close();
}

}