Put a cap on automatic discounts

We’ve introduced a 20% discount code for new users who register on our website. However, this code only applies to items that are not already discounted up to 20%. If a product already has a discount, the code will only cover the difference up to a maximum of 20%. For example, if an item is already 12% off, the code will add the remaining 8%. If an item already has a 20% discount, the code will not provide any additional discount. In all cases, the maximum total discount at checkout will not exceed 20%