Comma-separated list of columns to use as unique identifiers to find duplicates. Valid values: email, phone, linkedin_url, company_firstname_lastname. Defaults to "email" if not provided (backward compatible with existing behavior).
Note: The order in which you specify columns does not matter. The system will always check for duplicates in this priority order: email, linkedin_url, phone, company_firstname_lastname.
Examples:
- "email" - Use only email to find duplicates (default)
- "phone" - Use only phone to find duplicates
- "linkedin_url" - Use only LinkedIn URL to find duplicates
- "email,phone" - Use both email and phone to find duplicates
- "email,phone,linkedin_url,company_firstname_lastname" - Use all four columns
