Hi, Rachel! Cinderella's Royal Table is one of the most difficult reservations to get anywhere at Walt Disney World. In most cases, the restaurant fills up within hours of the 180 day reservation window.
When you make your reservation, you are required to give your credit card information so they can charge your card for the meal, even if you are on one of the dining plans. In this way, they can encourage guests who opt not to attend their meal at Cinderella's Royal Table to cancel their advanced dining reservations, so that the tables may be used for other guests who were disappointed not to find an opening.
When you go to the restaurant, your dining credits will be charged and your credit card refunded. You will not end up paying twice for your meal.
I hope you have an absolutely magical vacation, and have a wonderful experience at Cinderella Castle!