Originally Posted by
channa
Though not on the same callsign -- they'd have to modify that.
I've heard UA put a letter at the end of it sometimes.
EGF used to have 2 of the same callsigns in the air on opposite sides of the country all the time until they went from 3 digit operating flight numbers to 4.
ANZ does it almost daily with flight 1 in UK and NZ airspace at the same time.