Hm, I am a little bit confused. How are the amenities of luxuries now working? The civilopedia says one lux -> 1 amenity in 4 cities, that's it... And judging from the lets plays it looks like it is the case.
For reasons that are not at all clear to me, many in the community seem convinced that a extra copy of a luxury can be used to provide amenities for cities 5 through 8 (and that a third copy can support cities 9 through 12, etc.). I see nothing in the Firaxis livestreams or in the few preview vids by YouTubers that I've watched to support that view, but there is a lot of theory-crafting going on that depends on/assumes that is the case. If it is not the case, then extra copies of a luxury will be good for only two purposes: (1) trading with the AI (for a new unique luxury, or other goodies) and (2) serving as an "amenities insurance policy" (in case barbs or an enemy pillage your first copy of that luxury).