Collectors.flatMapping

public static <T, U, A, R>
Collector<T, ?, R> flatMapping(Function<? super T, Stream<? extends U>> mapper,
                               Collector<? super U, A, R> downstream) {
    BiConsumer<A, ? super U> downstreamAccumulator = downstream.accumulator();
    return new Collectors.CollectorImpl<>(downstream.supplier(),
            (r, t) -> Opp.ofNullable(t).map(mapper).ifPresent(s -> s.sequential()
                    .forEach(v -> downstreamAccumulator.accept(r, v))),
            downstream.combiner(), downstream.finisher(),
            downstream.characteristics());
}

List<Integer> actual = Stream.iterate(0, i -> ++i).limit(3)
        .collect(Collectors.flatMapping(i -> Stream.of(i, i), Collectors.toList()));
Assertions.assertEquals(Arrays.asList(0, 0, 1, 1, 2, 2), actual);