Originally Posted by
channa
Though not on the same callsign -- they'd have to modify that.
That is incorrect. Although UAL will usually put a suffix alpha character, it is common to operate two in the ATC system at the same time.
http://flightaware.com/about/faq.rvt#twoflights
Why does FlightAware show two airlines flights operating simultaneously with the same flight number? Is that possible?
Unfortunately, this is not a bug. It seems that some airlines occasionally have duplicate flight numbers in the air at the same time with different origins and destinations. When tracking a flight that is in the air twice, the arrival/departure history will be accurate but the position, speed, and altitude will be unreliable.