Can an airline operate with two flights in one day to the same destination with the same flight number for example
DUBAI to MUSCAT ETD 0400Z SSV 221
DUBAI MUSCAT ETD 1300Z SSV 221
These are not the same flights.
221 is not a flight number. Etihad between Dubai and Muscat operates 3 flights 382, 384, 388. But if the flight is delayed it can turn out that same flight on the same day leaves at 2 different times. See April 8: flightaware.com/live/flight/ETD221
I think according to IATA standard no flights should operative using the same flight number on the same day. If the flight is delayed by a day, the flight number should be changed by adding a suffix D like XX1234 to XX1234D.