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My brother bought a ticket directly from an airline office. He is a US citizen, but when he arrived at the airport, the airport staff offloaded him and gave the reason as his Pakistani visa was expired. When we came back to the airline office we asked why they did not check his visa before issuing the ticket. The airline staff said that this is not their responsibility. That it is always the traveler's responsibility. Can you please tell me if the staff is right?