Two (or more) contacts may have the same email address. Or, we might not know their email addresses.

Also, we have companies where multiple people have the same last name (e.g., because of family-run businesses).

When we add a new contact, we need to make sure it is not a duplicate.  

What is the best way to do this since none of the above fields -- last name, first name, email -- are not unique, but the combination is.  Checking on last name and first name would also suffice.

thank you.