Our system recognizes an EMAIL ADDRESS as unique.
So, if you are uploading a contact, e.g. Alex Meier with an email email@example.com and this same email (firstname.lastname@example.org) is already in your address book with e.g. Mr. and Mrs. Meier, the contact from the ADDRESS BOOK WILL BE OVERWRITTEN by the new content of your Excel Sheet.
FYI: This is "new" and was updated April 2017 to simplify the uploading process.