There is 5k resistor in cap on cx500 as stock, so your spark plug don’t need it. After a time it can be burned or somehow damaged by the age. Now you can buy a iridium plug with a 5 resistor built in. Iridium sparkplugs last more time and they have better spark.
Reason why sparkplugs have resistor is mainly, because it can produce radio frequency interference, which intefers electronic devices, stereos …
You don’t need 2 serial, so you can remove it from cap. You must put then a brass rod in, which is the same length as resistor.
