You can find submission duplicates by exporting your results from Paperform as a CSV and then importing the results to Google Sheets or Excel. Once in the sheet, choose a column that contains a unique attribute entered by the submitter (IP Address, email, name, etc.), and sort that column by A to Z or something similar. You should then be able to scroll through and spot any duplicates.
If you already have a submission in mind to search for, then you can simply type into the search bar of the Paperform dashboard (see below) for the user's name or email and all results containing this search will appear.