Many students check their credit card statement very soon after subscribing and notice two (or more) entries with the same price, causing students to believe they have been double-charged.
Payment processors (we use a widely-respected third-party payment processor) commonly put a pending charge on a card and then soon after, put an actual charge. The pending charge disappears in a day or two.
So if you see multiple charges, you might wait a few days and check again. Almost surely, you'll just see one charge at that time.
Of course, if you still see double charges after a few days, by all means email firstname.lastname@example.org. We think our content is well worth the price, but we won't charge you double for it!