A sales icon may appear on a product that is no longer on sale because a product that is discounted by the app gets duplicated. Product Discount is then unable to remove the discount on the duplicate product, and your store will think it's still on sale.
To fix this, create a discount with the duplicate product selected. Start and then stop the discount, when you stop it, the sales icon will be removed. If the item is still discounted, then you will have to manually change the product price back to what it was before the sale.
Visit Create a Discount with Product Discount for complete instructions.