We have an issue where we have individual customers (with email) and subsequently contacts of the same person (same email) and netsuite duplicate detection cannot compare two different types of records - contact vs individual customer, so I don't know how to alert us/prevent these duplicates. Does anyone know how I can go about solving?

customer record type and contact are two different types. having the same email is not considered dup

yeah, standard dupe detection won't work across different record types. You'd have to create your own dupe detection using a workflow or script