Originally Posted by ubernostrum

This is not quite true; as has been noted, AA has a habit of using the same flight number on a TATL and on a domestic flight scheduled to go after the TATL arrives, and there have been plenty of times when the TATL was delayed enough that it and the domestic flight were airborne at the same time under the same flight number.

This can confuse tracking websites like flightaware or flightradar, but is allowed by ATC.