So many buyers are saying the same thing that it's beginning to sound like a problem with eBay's system.
You will just have to check everything to find out where the error occurred.
Start with your purchase history in My eBay. Check that you didn't accidentally order two of the same - it happens. Also that you haven't overlooked the cost of postage and packing - again, it happens.
If this still shows payment due only for a single item, you need to log in to the applicable payment provider; this can be a credit or debit card, PayPal or PayPal credit, Apple Pay or Google Pay.
Doing the above should show whether eBay has mistanly charged you twice, or if the relevant payment institution has mistakenly duplicated the payment. Then you will know to whom to complain.
