They are not allowed to take open bottles; it all goes down the drain. I don't know if full bottles have to go back into inventory or if the crew can keep them if given as gifts.
As for bringing back excess liquor (US allows only one liter per person now) I just claim it all. Since duty is only a few cents a bottle, I have yet to have customs officer ask me to pay. Even if they do make me pay, it is cheap.
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