Why is there a need to pick an airline? Fight numbers, that I know of, are unique.
Tail numbers are unique; flight numbers are not.
It's quite normal, especially in the US
> United 1644 operates BDL-IAD-IAH-MCO, yet BDL-IAD operates as a 737-800, IAD-IAH operates as a 757-200, and IAH-MCO flies with a 757-300.
https://www.southwestaircommunity.com/t5/Southwest-Stories/T...
Looks like that specific flight number is no longer a multi segment.