import java.util.*;
import java.util.regex.*;
import java.lang.*;
import java.io.*;
 
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		String s = "This is a test\nAnd this is also a test\nAnd these are also tests\ntest\nЭто тест\nЭто также тест\nИ это также тесты";
		String rx = "(?sU)\\b(\\w+)\\b(?=.*\\b\\1\\b)";
		String[] results = Pattern.compile(rx).matcher(s).results().map(MatchResult::group).toArray(String[]::new);
		//System.out.println(results);
		System.out.println(s.replaceAll("(?U)\\b(?:" + String.join("|", results) + ")\\b", "_$0"));
	}
}

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