With this code I got the same expected results except for a mysterious character at the end of the last test case. It is neither a whitespace or unprintable char. It makes no sense for me.

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 = input.replaceAll("(?i)"+regex,"$1");
                //m = p.matcher(input);
            }
            
            // Prints the modified sentence.
            System.out.print(input+'\n');
        }
			
        in.close();
    }
}

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 = input.replaceAll(m.group(), m.group(1));
        }

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

my solution: 1. \b - is a word delimiter 2. \w+ - any word, letter, digit or underscore 3. \s+ - blank spaces 4. \1 - back reference (anything captured by (\w+))

public static void main(String[] args) {

        String regex = "\\b(\\w+)(\\s\\1)+\\b";
        Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE /* Insert the correct Pattern flag here.*/);

        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 = input.replaceAll(m.group()/* The regex to replace */, m.group(1)/* The replacement. */);
            }
            
            // Prints the modified sentence.
            System.out.println(input);
        }
        
        in.close();
    }

Is this what we really want? I think that this paragraph should be improved

3. Write the two necessary arguments for replaceAll such** that each repeated word is replaced with the very first instance the word found in the sentence**. It must be the exact first occurrence of the word, as the expected output is case-sensitive.

public static void main(String[] args) {

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

    Scanner in = new Scanner(System.in);
    int numSentences = Integer.parseInt(in.nextLine());
    int i=0;
    while (i<numSentences) {
        String input = in.nextLine(); 
        Matcher m = p.matcher(input);
        input = m.replaceAll("$1");

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

    in.close();
}