When you bulk upload you are presented with three options.
If a user has the same email address as someone in this workspace then we will not upload the contact.
It's also worth noting that when you upload via the "Enter Emails Manually" option, we will automatically upload via the "Skip Duplicates" options.
Overwrite All Fields
If the email address matches an existing contact in your workspace we will overwrite the contact with the new data.
Only Update Empty Fields / Missing Fields
If the email address matches an existing contact in your workspace we will only update the fields that are empty.