JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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Yes, you can integrate your JotForm form to Google spreadsheet by following this guide: https://www.jotform.com/help/228-How-to-Integrate-Form-with-Google-Spreadsheet. It should automatically load all the existing submission of your form in the spreadsheet, now every time there's a new submission, it will automatically be added in the spreadsheet.
I hope that helps.
How does it handle duplicate data?
It will export each submission from your submissions page into a separate row in the integrated Google Spreadsheet. In addition to integrating your form with Google Spreadsheet, you can also manually export your submissions into Excel format at any time:
The exporting process, either with Excel fiels or with Google Spreadsheets, doesn't add any additional handling / processing of possible duplicated data.
If you have duplicated submissions on your form's submissions page, you will have duplicated entries / rows in the Google Spreadsheet and any exported Excel files. Each submissions is in its own row in the spreadsheet.
Please let us know if you need further assistance or clarifications, and we will be happy to help.