Sorry if I didn't read preceding posts closely enough. "Duty free" refers to whether you paid duty. If you bought the stuff outside an airport duty-free shop, then you paid duty and your limit is different. Personal experience: 156 liters of duty-paid beer which caused multiple head explosions and a lot of general embuggerance until all parties concurred that no extra duty was payable.
In general, the rules are not asking the question "have you paid our duty?" but rather "have you paid duty anywhere?" Affirmative to the latter, then things are much more relaxed although you need to be patient with the customs people sometimes.