Looking at the database from the sample link you sent< i assume you are using that exact database with same field names. that database is using a unique field called 'customer_ID'. This ID is already being created as each excel file is being uploaded.
so all you needed to do is create a query to get the recordset of that viewer's id:
SQL = "SELECT * FROM customer2 where Email = '"&Email&"' "
Assuming you are using their email as a username.
as for duplicate files, Your upload system might automatically write over files with same name avoiding a duplicate. do a test first and confirm this.