Hi Catarina,
Standard behavior is done in FM HR_ERC_DUPLI_CHECK_EXT and here we check first the email address. So if that exists, we check if a candidate exists for this BP with this email address. Then it checks for first and last name.
If you want to overrule that behavior you will need to implement an own BAdI implementation to avoid that the fallback class is called.
Regards,
Nicole