There is war weariness, though I don't know what its consequences are. Most of my wars have been defensive or strategic so far, not domination.
As for going wide, luxury resources can cover four cities but they can't cover the same city. So if you have two identical resources and 4 cities, each gets 1 amenity. If you have two identical resources and 8 cities, each gets 1 amenity. If you have two different resources and 4 cities, each gets two amenity. If you have two different resources and 8 cities, each gets one amenity. Amenity are evenly spread out amongst cities (so 5 cities with 1 luxury resource would have amenities of 1, 1, 1, 1, 0). This is why you should be careful trading away extra resources, especially if you're a warmonger. Also why trading for different resources isn't always going to net you a plus.