So what happens if the wrong a/c squawks ident, and the crew of the right a/c nearby has missed the ATC request to do it?
..then ATC will see that the wrong a/c is identing because it will be the wrong code? Whilst it is mandatory in blighty in the US it is not mandatory to read back transponder instructions in airborne comms:
"Pilots of airborne aircraft should read back those parts of ATC clearances and instructions containing altitude assignments, vectors, or runway assignments as a means of mutual verification." FAA AIM.
I was taught - when in the US - to simply go ahead and set the code and ident rather than wasting a call reading back something that ATC are about to verify on their screen anyway. Nonetheless it seems that many - perhaps even most - pilots in the US do readback.