Three weeks ago, on NYE, the guest in front of me at an Embassy Suites was cut-off (half an hour into the free cocktails). He couldn't have been more than 21-22 or so, and of course he was belligerent about it. Those are uncomfortable situations in the lobby of a big hotel, where they can summon a couple of big bouncer-types in an instant (and did just that, which quickly had the desired calming effect on the guest).

Watching that unfold in front of me gave me a renewed appreciation for the alcohol balancing act performed every day by FAs. In the air. Where they can't simply eject you back to your guestroom if you've over-imbibed and become obnoxious about it. Oh, sure, they can summon the LEOs upon landing, and have the drunk arrested, but nobody really wants to do all that.

All that said, I've been served doubles (even in premium cabins) all my adult life, usually without asking for them. Makes sense that the flight attendant training would address serving multiples (presumably, fewer disruptive drunks and fewer drinks served equals smaller expense) and that many flight attendants probably ignore that rule and substitute their best judgment. In domestic economy, where alcohol is still a revenue source, selling multiples would seem to maximize the revenue. But it also leads to less monitoring of the passenger's consumption and makes it easier for passengers to share with underage companions; think March and April flights where the person with proof of age orders doubles and their companion just orders (free) mixers.