Use Shopify discount code or Automatic Shopify collection of non-discounted products

Alternative 1 (recommended for discount codes): Create a Shopify automated collection that contains only products without a compareatprice discount (or where compareatprice equals price). Then create a Shopify native discount code targeting that collection so the code will not apply to products with a compareatprice discount.

Alternative 2 (if you can accept automatic discounts instead of codes): Use an automatic discount (Shopify or the app’s automatic discount functionality) and ensure it targets only the explicit products/variants that are not already discounted (maintain the product selection manually or via a Shopify collection). Note: Automatic discounts are not the same as Advanced Discount Code campaigns and have different behavior regarding stacking.