Thank you for reaching out with your question. When you create a discount code it is not possible to automatically exclude all products currently listed with a sale price, however you can specify that the discount code only applies to certain products and collections:
Using these options, you can select the products or collections you want the discount code to apply to, and can therefore exclude all products already on sale by not selecting them or adding them to the collection(s) you choose here. I hope this explains things but please let me know if you have further questions.
Victor | Shopify Social Care