Credit card companies are usually pretty vigilant about fraud, and sometimes a little too much, but at times, transactions can be repeated two or more times on the your statement accidentally, or intentionally for that matter. It’s always a good idea to keep all your travel receipts and documentation as proof until the transactions have been cleared and you’ve reviewed your statement for inaccuracies. Same goes for mileage, but boarding passes aren’t such a problem anymore with mobile technology. But still, if you’re using old-fashioned hard-copy passes, keep them until your mileage has been posted.