What about one land and one other for that sake? (like settler, worker, missionary, sea, air or whatever) but Generally, I'd much rather see a Unique national wonder for each civ, quite easy to find for every civ, you can give them much diverse effects and it would be an addition, not a doubling. Or else, they could add a unique ability, make it flanking III promotion for the Mongols, thermae unique building for the Romans, Super Caste-Civic for india, the Sphinx for Egypt, etc. you know something unique for every civ that doesn't have to fall in the same category. That way you can add second uniques but you don't have to invent one for some civs (2uu for inca? you'd be hardpressed, but you can easily give them a bonus to roads, mountains or whatever...)
But I will not mourn if they add neither (which is probable).
mick