this is simply
~~~scala
underlying.map(...).concatAll
~~~
and it could be further simplified to the following given mapFuture:
~~~scala
underlying.mapFuture { p =>
  val (ductIn, tailStream) = Duct[T].drop(1).build(mat)
  Flow(p).tee(ductIn).take(1).map(_ -> tailStream).toFuture(mat)
}
~~~