Usually the error “Duplicate eBay Checkout SessionId found. Please process one purchase at a time” happens when eBay sets a temporary purchasing limit for your account (buying limit). This usually happens when there is a spike in activity (orders, amounts spent, etc.) and is automatically reset . To speed up the reset of this limit, you can read this article https://pages.ebay.com/help/sell/limits-on-buyers.html or contact eBay support.