FixMe.java

package it.test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;

public class FixMe {

    public static void main(String[] args) {
        Integer[] integers = new Integer[]{1, 2, 2, 4, 5, -1, -2, 23, 42, -1};

        final Collection<Integer> result = duplicateSome(new ArrayList<>(Arrays.asList(integers)), 2);
        System.out.println(result);

        final Collection<Integer> result2 = duplicateSome(new ArrayList<>(Arrays.asList(integers)), null);
        System.out.println(result2);

    }

    /**
     * Returns a new Collection of Integers where all the numbers that are multiple of test are duplicated
     * @param numbers A list of Integers
     * @param test The MCD. null means "do nothing"
     * @return A new {@link Collection} that satisfies the requirements
     */
    private static Collection<Integer> duplicateSome(Collection<Integer> numbers, Integer test) {
        for (Integer number : numbers) {
            if (matches(number, test)) {
                numbers.add(number);
            }
        }
        return numbers;
    }

    private static boolean matches(Integer number, Integer test) {
        return number % test == 0;
    }
}