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Is it allowed to purchase alcohol at a duty free store on the way out of the country, and bring it back when I return? Assuming I bring less than the personal exemption.

Specifically, I'm asking about crossing the U.S.-Canada border by bus.

I understand that U.S. citizens can bring into Canada

1.5 litres of wine, or 1.14 litres (40 ounces) of liquor, or 24 x 355 millilitres (12 ounces) cans or bottles (8.5 litres) of beer or ale.

and, after 48 hours, can return with

Purchases [that] may include 1 litre of alcohol, 200 cigarettes (1 carton), and 100 cigars.

Further down on that page, it says

Liquor, including spirits, wine and beer, in Canada is significantly more expensive than in the United States, so Americans going to Canada for a visit may want to stop at duty free for liquor that they will consume while in Canada.

Does this mean that I must consume all of the alcohol I purchase at duty free before crossing the border? If I declare it, will the bottle be labeled in a way that a border officer would even be able to distinguish it from another bottle purchased in Canada?

Source.

The U.S. CBP info page doesn't clarify. To be clear, I will be in Canada for multiple days for a legitimate pleasure trip.

Does this mean that I must consume all of the alcohol I purchase at duty free before crossing the border?

No. It means that if you're planning to drink alcohol in Canada, it could be a good idea to buy some at the DF store.

will the bottle be labeled in a way that a border officer would even be able to distinguish it from another bottle purchased in Canada?

Yes, but more accurately, it will not be labeled like a bottle bought in Canada -- it will miss the tax sticker and importer's info. But that doesn't matter. If the alcohol you are carrying is within the limits, you don't need to declare it.