SnapRetail will not allow an email address to be added to your account more than once. If the same email address is uploaded more than once, the information associated with the email address will update.
This means, that if a customer email address is added including first name, last name, phone number information to your account and you later upload the same email address with less or different information, the information will update to only include what is entered during the second upload.
Alternatively, if a customer email address (or a spreadsheet) is uploaded and added to a list and the same email address (or spreadsheet) is added again to a second list, the customer will appear on both lists.
Adding customers to multiple lists does not "duplicate" the email address and is only accounted for once.