Napster may check the validity of your credit card by using an authorization hold. The amount held is temporary and not a charge. Your credit card company controls the amount of time it takes to release the hold. The hold may temporarily reduce the amount of available credit on your card.
Napster charges a monthly fee per subscription. If you see more than one monthly charge on your credit card, you may have more than one Napster subscription. See this article for steps on canceling Napster.
A temporary line item may appear on your credit card if you change from one subscription level to another. However, you have not been charged twice. If you move to a lower priced subscription, you will receive a prorated credit. If you move to a higher priced subscription, you will be charged the difference.