I finished all adventures but don't have 6 of a kind of citizens, cause I started playing the game, when citizens weren't in the game. They were introduced later and I didn't get them as an reward afterwards.
I think best think would be to create a buying or exchange system for cards, cause also for other cards there is the need to get the 7th and 8th one to upgrade them.
For example a 10:1 exchange rate for cards of the same rarity could solve the problem...