Discount on discounted products

Hello,

I’m trying to apply a percentage discount to a category and everything seems to go right but if I’ve got some items in this category with a previous discount applied (filled in the “Sale Price” field of the woocommerce product data – General tab), I get the item with two discounts applied.
My question is, is there a way to apply the discount to the regular price and not to the sale price?

Thanks in advance!