This Shopify Script provides a discount when the eligible item is found and the cart has more than one item.

ELIGIBLE_PRODUCT_ID = 111111111 # Specify which product ID creates a discount
DISCOUNT_AMOUNT = 0.2 # Set the discount amount here

has_product_in_cart = Input.cart.line_items.any? do |line_item|
    line_item.variant.product.id == ELIGIBLE_PRODUCT_ID
end

if has_product_in_cart && Input.cart.line_items.length > 1
    Input.cart.line_items.each do |line_item|
        discounted_price = line_item.line_price * (1.0 - DISCOUNT_AMOUNT)
        line_item.change_line_price(discounted_price, message: "#{DISCOUNT_AMOUNT * 100}% off!")
    end
end

Output.cart = Input.cart