Hello,
I understand you would like to provide a coupon to be applied only once, if the customer selects the same product multiple times, then the coupon will not deduct from either one.
I think you can consider the Smart Coupons premium extension, https://woocommerce.com/products/smart-coupons/, we do not provide support for premium extensions on our forum, but you have the chance to make pre-sales questions here: https://woocommerce.com/my-account/create-a-ticket/ to see if it will fit your needs.
Premium extensions like Smart Coupons offer a refund policy (The timeframe is 30 days after the charge, no questions asked). You can read more about our refund policy on our website here: https://woocommerce.com/refund-policy/
The www.ads-software.com plugin repository have other alternatives, you can explore more plugins to help you to manage coupons in this search.
I hope this information is helpful to you.