"Three times in the past ten days I have been double-charged for
journeys involving this change. ... Raising this with the Oystercard
refund department, I was at first told it "could not happen". But when
they investigated my journey record online they were forced to admit it
had indeed happened. I've now had £3 refunded to my card. Ticket
office staff at Northfields have told me I'm not the only "victim".
There have recently been numerous incidences of Oyster overcharging both
at Hammersmith and Kings Cross."

Anyone else suffered from this? It obviously pays to keep a close eye
on your PAYG balance when using these stations.

I've used Hammersmith fine without any ill effects. Sometimes I think
people get concerned when they see the balance displayed on the exit
gates - I think credit is deducted upon leaving one station but not
"refunded" upon entering the other, giving the impression that they have
been double-charged (the journey resolves fully at the final station).

However, if that person got a £3 refund, it seems likely that they were
genuinely been overcharged. They only explanations I can think of are
either a missed touch-in or touch-out at one of the four validation
points, or an excessive amount of time spent mid-interchange (e.g. doing
some shopping in between changing lines at Hammersmith). I don't know
what the time-out is for out-of-station interchanges - thirty minutes
rings a bell somewhere. I've managed twenty minutes between validations
at Marylebone en route to Amersham before.

This problem usually arises when a touch out has been missed. Oyster
can cope with an entry followed by two exits, as it just extends the
journey. However two entries followed by one exit will lead to an
unresolved journey.