The rules are a lot more complicated than you are trying to make out.
Have a read of the online Routeing Guide and it will make sense.
There is a fares rule, and there is a doubling-back rule, but neither
are as you state.

I'm sorry, this was a misunderstanding on my part. Let me explain the
situation fully:

DOUBLING BACK
Doubling back (passing through the same station twice between origin
and
destination) is forbidden, but with three exceptions. Doubling back is
permitted if
1. an easement allows it
2. it is for the purpose of changing trains inside a routeing point
group
3. it occurs on map ZZ or in combination with map ZZ to reach a
station the sleeper
train does not stop at.

THE FARE-CHECK RULE
The fare-check rule is that a journey is allowed by an origin or
destination routeing point if the fare for the whole journey via that
routeing point is not less than the fare from the origin to the
destination routeing point and not less than the fare from the origin
routeing point to the destination.


To explain in more detail,
Paddington = London Group Routeing Point Member
Clapham Junction = Clapham Junction Routing Point

Permitted Routes:
LB: http://www.atoc.org/rsp/_downloads/R...s/Map%20LB.jpg
LK: http://www.atoc.org/rsp/_downloads/R...s/Map%20LK.jpg
PD: http://www.atoc.org/rsp/_downloads/R...s/Map%20PD.jpg
SC: http://www.atoc.org/rsp/_downloads/R...s/Map%20SC.jpg
WX: http://www.atoc.org/rsp/_downloads/R...s/Map%20WX.jpg

None of these routes utilize Paddington Station, therefore travel from
any train leaving Paddington is not permitted. Therefore, the London
Underground must be used [1]. However, travel from Charing Cross,
Waterloo, Victoria, Canon Street, London Bridge or Any Thameslink is
permitted. As Reading is not a permitted routeing point, the Fare-
Check rule does not come into play.

[1] "Sometimes the Fares Manual will show an “Any permitted� fare but
without the via
London, Maltese cross âœ* symbol. Reference to Section C may show via
London to
be a permitted route for this journey and in such instances travel via
London to
include cross-London transfer would be permitted."

Which explains why the ticket barriers opened. It doesn't however say
why they didn't open on the way back