I agree that this should be automatic or the default. Nice work around, but this is a good one for the wishlist.

I found a solution: An autoplaylist with a search criterion "playlist is ..." pointing to child autoplaylists. With the use of "Match all/any" it can work.
I made a few tests, and it seems that the resulting playlist has no duplicates.

I wish that any parent Playlist would display the context of its child nodes (no duplicates). I think that a skilled script writer can do it in no time.

Another option is to write complex filters, e.g. [color=#0000FF]Rating=5 stars AND Date between(1955;1963) AND (Genre equals 50s OR Genre is unknown)[/color].