The Discard Subsequent Rules option is only applied if the pricing rules have different priorities. If two pricing rules have the same priority, the discount from both rules will be applied.

For the 'Discard Subsequent Rules' option set to 'Yes' to work effectively, each pricing rule must have a defined priority. When multiple discount rules can apply to the same product, Magento evaluates them in the order of their priority values. If a rule with 'Discard Subsequent Rules' set to 'Yes' is applied, any subsequent rules with lower priority (higher priority number) will not be applied to that product.