Originally Posted by
PDXNRTLHR
As far as I know, not necessarily. The flight number they use for ATC purposes is not necessarily the flight number they use for marketing, etc. For example, they could add an "A" to the second flight.
Correct. Operationally they will simply call one of them a different # (or 512A, as noted) to avoid confusion. This is an internal designation for the airline/feds, the flight # won't (or rather, shouldn't) change from a passenger's perspective.