n message , Dowager
writes

Question: Surely you can't be sold a valid ticket (whether pre- or post-
journey) AND also be issued with a PF for that same journey?

Yes: http://www.ircas.co.uk/penaltyfares/pf_faq.htm

If you board a train without a valid ticket for your entire
journey, or a valid Permit to Travel, you may be charged a
Penalty Fare of £20 or twice the full single fare (whichever
is
the greater) to the next stop, plus the full single fare to
complete your journey.

--
Paul Terry

As a lawyer I would point out the use of the word 'may', so discretion
is allowed. Also, the regulation appears to imply that it only applies
to travelling revenue officers (or whatever they are called) as the
penalty is either £20 or twice the single fare etc....it is arguable
that it does not apply once the journey is finished.

Cliff