I'm flying from Indonesia to Boston, USA (flight route: CGK-AUH-DUB-BOS). Since both airports in AUH and DUB provide the pre-clearance service, how do I know in which airport I should go through the pre-clearance process?
Your flight from AUH to DUB is shared by many passengers not traveling to the US, so preclearance in AUH is not possible. Everyone on the plane must go through preclearance, and the flight must proceed directly to the US. So preclearance can only be done at the last airport you touch before flying to the US, if it's available there.