@Test
public void shouldDuplicateElementsInAList() throws Exception {

    List<String> duplicates = P14.duplicate(Arrays.asList("a", "b", "c", "d"));
    assertThat(duplicates, hasSize(8));
    assertThat(duplicates, contains("a", "a", "b", "b", "c", "c", "d", "d"));
}