score:3
There are a finite number of flight designations, particularly as the legacy systems behind a good deal of scheduling and reservations are limited to a two-character airline code and a four-digit flight code (XX0000).
Over the last two decades there has been an extraordinary expansion of codeshare agreements, not just for regional carriers, but through inter-airline agreements and the global airline alliances. AA1 is not just AA1. It is simultaneously BA2430 and QF3100, and also QR7770 and TN1101, and also AT5034 and B64000. Those numbers come out of each of those carriers' "inventory" of available flight numbers.
There has also been considerable industry consolidation. Although most of the European carriers continue to operate separately, the North American carriers have opted to unify, so while the overall number of flights has probably decreased (I don't have the numbers in front of me, but capacity has been noticeably reduced over the last decade), there are fewer airline codes to split the designations among.
As a result, there has been a surge in demand for flight numbers, and scheduling two flights with the same number is one way an airline can "conserve" them. Short regional flights from a hub, for example, might use the same number on the outbound and the return.
It has long been possible that a delayed flight and the next day's flight might be operating at the same time, with the same origins and destinations, and in the same air traffic control sectors at the same time, and so operationally I expect there is a way of handling this, though I am not familiar with the details. But under regular operations, I don't think either the airline or its regulators would want to risk having two flights with the same number aloft in the same sector at the same time.
Upvote:8
Sometimes its just a marketing trick.
Some airlines simply combine two arbitrary routes through a hub, give it the same flight number and the market it as a "direct flight" from A to B.
Example: I booked a "direct flight" on United from Boston to Vegas and it turns out it had a stop in Denver. It was NOT the same plane, NOT the same crew, NOT the same gate. My outbound from Boston was delayed, our flight to Vegas had already left, so I had to spend the night in Denver.
So in this case the only difference between a "direct flight" and a normal connection was that both legs have the same flight number.