Duplicate in String

package com.company;

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

public class Algorithm {
    Set<Character> duplicate(String string) {
        if (string.isBlank() || string.isEmpty()) {
            throw new RuntimeException("Invalid String");
        }
        Set<Character> characters = new HashSet<>();
        char[] chars = string.toCharArray();
        for (char aChar : chars) {
            if (string.replaceFirst(String.valueOf(aChar), "").length() != string.replaceAll(String.valueOf(aChar), "").length() && !characters.contains(aChar)) {
                characters.add(aChar);
            }
        }
        return characters;
    }
}

package com.company;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

public class AlgorithmTest {
    @Test
    void duplicate1() {
        Algorithm algorithm = new Algorithm();
        Set<Character> duplicate = algorithm.duplicate("aabbc");
        Assertions.assertEquals(new HashSet<>(Arrays.asList('a', 'b')), duplicate);
    }

    @Test
    void duplicate2() {
        Algorithm algorithm = new Algorithm();
        Set<Character> duplicate = algorithm.duplicate("sshhhrriiikkaaannttt");
        Assertions.assertEquals(new HashSet<>(Arrays.asList('s', 'h', 'r', 'i', 'k', 'a', 'n', 't')), duplicate);
    }

    @Test
    void duplicate3() {
        Algorithm algorithm = new Algorithm();
        Set<Character> duplicate = algorithm.duplicate("madam");
        Assertions.assertEquals(new HashSet<>(Arrays.asList('m', 'a')), duplicate);
    }
}

plugins {
    id 'java'
}

group 'org.example'
version '1.0-SNAPSHOT'

repositories {
    mavenCentral()
}

dependencies {
    testImplementation('org.junit.jupiter:junit-jupiter:5.6.2')
}

test {
    useJUnitPlatform()
}

package com.company;

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

public class Algorithm {
    Set<Character> duplicate(String string) {
        if (string.isBlank() || string.isEmpty()) {
            throw new RuntimeException("Invalid String");
        }
        Set<Character> characters = new HashSet<>();
        char[] chars = string.toCharArray();
        for (char aChar : chars) {
            if (string.length() - 1 > string.replace(String.valueOf(aChar), "").length() & !characters.contains(aChar)) {
                characters.add(aChar);
            }
        }
        return characters;
    }
}

package com.company;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

public class AlgorithmTest {
    @Test
    void duplicate1() {
        Algorithm algorithm = new Algorithm();
        Set<Character> duplicate = algorithm.duplicate("aabbc");
        Assertions.assertEquals(new HashSet<>(Arrays.asList('a', 'b')), duplicate);
    }

    @Test
    void duplicate2() {
        Algorithm algorithm = new Algorithm();
        Set<Character> duplicate = algorithm.duplicate("sshhhrriiikkaaannttt");
        Assertions.assertEquals(new HashSet<>(Arrays.asList('s', 'h', 'r', 'i', 'k', 'a', 'n', 't')), duplicate);
    }

    @Test
    void duplicate3() {
        Algorithm algorithm = new Algorithm();
        Set<Character> duplicate = algorithm.duplicate("madam");
        Assertions.assertEquals(new HashSet<>(Arrays.asList('m', 'a')), duplicate);
    }
}

plugins {
    id 'java'
}

group 'org.example'
version '1.0-SNAPSHOT'

repositories {
    mavenCentral()
}

dependencies {
    testImplementation('org.junit.jupiter:junit-jupiter:5.6.2')
}

test {
    useJUnitPlatform()
}