If a contact already exists in the participant's address book and an upload is performed on the same email address, the system will recognize them as potential duplicates and prompt the user for a resolution path, either by choosing the Add, Update or Ignore option.

A potential duplicate is found when attempting to upload information about a contact whose email address is already in the Address book in this Participant Center.
From here, you will be able to easily perform updates to existing contact records without creating duplicate contact entries.