MIG wrote:

Given that there are restrictions that prevent any kind of successive
passes at the same gateline, there is still nothing to be gained by
passing the ticket through an extra time, and no need to program
against it, because it's already ruled out by simpler restrictions.

That depends on the exact restriction.

If the restriction is that a single card cannot be used twice in rapid
succession at the same gateline, then you are correct. But that is
/not/ the restriction that is actually enforced, since the system /will/
permit an entry followed immediately by an exit.

If the restriction is that a card that has most recently used to enter a
station cannot shortly thereafter be used to enter that same station,
then it prevents simple passbacks but not zig-zags, since with a
zig-zag, the most recent usage was an exit, not an entry.

Incidentally, I have been told that WMATA (the Metro in the Washington,
D.C. area) does /not/ have zig-zag restrictions on its unlimited cards.
--
David of Broadway
New York, NY, USA