// being true.
    Pattern pattern = Pattern.compile("Multiple entries with same key: four=(.*) and four=(.*)");
    assertThat(expected).hasMessageThat().matches(pattern);
    Matcher matcher = pattern.matcher(expected.getMessage());
    assertThat(matcher.matches()).isTrue();
    assertThat(matcher.group(1)).isNotEqualTo(matcher.group(2));
  }