It is not a double payment but rather a pre-authorization. It is important to note that a pre-authorization charge is not the final payment, but rather a temporary hold on your payment card. Once your order is shipped, the pre-authorization hold is released, and the actual charge is processed.
Your order may be shipped in multiple packages. In that case, multiple charges corresponding to each package appear on your account. The amount of each charge will add up to the total amount displayed on your order confirmation.