You can't. When someone downloads and fills-in the form they are working on
their local copy, on on the file that's on the server. You have no way of
knowing how many copies of the file were already submitted and no way of
preventing duplicate codes.
Thank you so much for your answer.
Well Do you have different any idea? How to make it. Is it possible to do PDFs or different way.
You can number the files that you receive (on your end, on on the user's
end), or you could use random numbers instead of sequential ones, or you
can use a web-based form instead of a PDF.
Ok I get it but I think this form look like very hard for me. Anyway thanks for your attention. Have a nice day.