Don't confuse duty with flight duty period. You can operate to a destination and have an FDP of say 14 hours. There is no limit to duty if you DH back. Duty is just working hours. A friend of mine had to operate Brisbane-SIN then DH back SIN-AUH last year when the 777 took over from the 330.
He asked RH about it who just giggled and said the above "Duty is not the same as FDP".
This is how our management plays guys, don't bend over for them. Do not go into discression, don't work on days off. It's the spineless types that let them no they can screw us.