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Imported alcohol in Indonesia is very expensive because of taxes.
If you bring your own, you have to put it in checked baggage so you run the risk of it getting broken. As Matt said, they have a 1 liter/person limit.
Yes, if you connect through Singapore you can pick it up at a duty free shop in Changi airport.
I connected once through KL and was able to do that, but it still cost more than in the US.