Not many cities have a multi dimensional portal. But the city of Esmeralda possesses one. With this portal Esmeralda can see itself in every other dimension it exists in. Esmeralda’s academia and scientists use this portal to look at its city and all its possible outcomes, so as to recommend changes to their city ensuring all the mistakes made in the other versions of Esmeralda are not repeated.
But every other version of Esmeralda also has a portal and does the exact same thing, using the portal to look at the other Esmeralda’s. So rather than creating the perfect city, Esmeralda is simply copying all the replicant cities. Each city that with every success, also has a flaw. With each Esmeralda copying the other, there will eventually be one uniform Esmeralda, with the same flaws and successes in every dimension.
There is a similar city in Italo Calvino’s Invisible Cities, but I feel this one is different from that one. If it is too close to the original then please could you let me know. Its been awhile since I have read it.