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 enter this information in the search bar on the left side of your submissions page (such as the respondent's name or email address). Then all results containing this data will appear.

Gotchas
- You cannot search for form responses across forms, or from your main Paperform dashboard.