I upgraded to cups 1.1.23-r8 from 1.1.23-r1 and after doing so when I print to my hp laserjet 8000 it always prints duplex. The ppd shows a bunch of disable options but it keeps doing it on every job. I then downgraded to cups 1.1.23-r1 and it is working again (working means, not printing in duplex).
you might want to try out with cups 1.2.2 and file a bug upstream when it does not work there.
please test with ~arch version 1.2.2