Since add_selected_items action doesn't respect space_add_mode option, maybe we should have another option for choosing add strategy when any of the songs being added to a playlist are already in it. I find it weird that option contains a keybind in it's name. I use space to pause songs.

I'm not sure how to call that new option, maybe something along the lines of add_selected_items_mode, it could have two modes: noop/prevent_duplicates.