Fix validation rule on a Woocommerce field.

I have a field on https://node5040.myfcloud.com/~examenesadomicil/producto/pcr-a-domicilio/ named RUT, that has a validation function. This works fine, but when user select more than 1 quantity, this custom field is duplicated but validation rule doesnt work for new field duplicated. I need to make it work for all duplicated RUT fields.