No, you can't duplicate your eBay products/variants.
Reusing the Printful SKU product/variant is not possible. This means that you cannot duplicate a listing on eBay because it will use the original SKU record, instead of generating a new SKU. As stock and product variants are defined by their SKU number, it has to have a unique combination, otherwise it cannot be duplicated.
However, you can Duplicate your existing products to list products with similar variables (i.e., sizes, colors, branding options) on Printful. Edit the copy to change or add another design or make changes to selected variables. Learn how to do it here.