Originally Posted by
cbn42
I think using two very different routes for the same number, several hours apart, would be the least confusing. For example, if I want to know what time my LAX-DFW flight departs, and I type in the flight number and find MIA-JFK, I will immediately know it's the wrong flight and keep looking. But if I find a flight that is LAX-SFO, or DFW-LAX, then if I'm not paying careful attention I might go to the wrong gate at the wrong time.
A particular flight number can only occur once per day in the ATC. (In cases where a flight is delayed past midnight that will cause a flight to get a new number in the ATC though that change will probably not be public facing.)
The out-and-return with the same flight number is not one flight number N from A to B and then another flight number N from B to A. It is one direct flight number N from A to A via B, just like you can have one direct flight number M from X to Z via Y.
This causes no confusion to normal people who would never actually book a direct fight from A to A. It only causes problems for a mileage runner trying to do a quick turn.