> What if someone else invents the same thing before the patent expires?

Government will offer you a nice sum of "keep quiet money" and an NDA, you're getting hit with NSLs, your patent is outright seized by the state, you're getting threats e.g. of tax or immigration checks... the ways government can intimidate you are endless.

For the obvious, legal, way see https://en.wikipedia.org/wiki/Invention_Secrecy_Act

So basically you're saying that any independent invention of the same thing is also classified.

As soon as you try to patent it, USPTO will look up if there's a similar thing - and if there is and it is "red-flagged", you're screwed...

But suppose you DON'T try to patent it. You just invent it and try use it or sell it. What then?