Southampton Central and Southampton Airport Parkway are both in the same routeing group, so doubling back is normally permitted between stations within the group (for interchange purposes only).
Hang on ... I think this answers a question in another thread. Trowbridge and Westbury are in the same station group (I just checked!) so that Melksham - Bradford-on-Avon / Bath Spa / Bristol Temple Meads etc passengers can double back at Westbury. The context of this was for customers in Wheelchairs which (platform to platform) at Trowbridge is all around the streets, but Westbury has lifts. Have I got that right?
That is my understanding of this text at the bottom of Page F8 in "The Routeing Guide in Detail":
6. Journeys may not double back except between stations which are members of a routeing point group for interchange or unless an easement permits it.
This can also be confirmed by using journey planners, but adding a Via station (see attached image).