There are three deduplication options that you can choose from when uploading contacts. They are:
skip
overwrite
update_missing_fields
Here are the difference between these options.
skip
If a user has the same email address as someone in this workspace then we will not upload the contact.
overwrite
If the email address matches an existing contact in your workspace we will overwrite the contact with the new data.
update_missing_fields
If the email address matches an existing contact in your workspace we will only update the fields that are empty.