How to add another field for duplication check in Contacts Module?

When a contact record is created, it checks if with same first_name and last_name already exists. I want to add another field for validation check along with first_name and last_name. (Account record that is being linked.)

How may I achieve this?

@pgr I would love to see some guidance from you. Thank you.

I don’t really know the answer, but there are 4 links about validation that I have bookmarked here:

Maybe try looking into those…