That's the key - if it was done once, it can be done again, and likely it's going to be significantly cheaper/easier because it's known it can be done. We see this from olympic records (e.g., the 4 minute mile was a "barrier" until one day it was passed and suddenly a bunch of people passed it).

Of course, doing it "legally" is another question - someone in the US trying to replicate would likely run into patent and other issues.

But a top-secret Manhattan-style project done by the US or China? definitely doable, and if you add spy-shit in, perhaps even faster.