I don't know why no one has suggested this but you could use Javascript to disable the submit button. Technically they could still do multiple submits by ignoring/disabling the Javascript but that does not appear to be your real concern at the moment.

In reply to Re: (CGI) Prevent a reload from resubmitting a form by Herkum
in thread (CGI) Prevent a reload from resubmitting a form by fmk

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