Well, let’s analogize:
our abilities in nanotech are supreme, and we get all the industrial processes right to manufacture everything in the proper way to produce a clone.
Now:
Across the street, two cars are manufactured and come out looking identical.
The two humans are identical as long as they do nothing.
The two cars are identical as long as they do nothing.
As soon as the car leaves the parking lot, it has a distinct identity. Hit by different pebbles on the road, forming its uniqueness.
As soon as the clone has its first thought, it’s already getting hit by different pebbles in the road (sensory data) dinging its brain and causing it to respond uniquely.