Can something like this be done with two surveys? First would be register-survey having a question of type equation using rand() and name 'foo', and it's end page would be 'http...realsurvey...&code={foo}'. Then after survey is done we could check if all answers are made in given time limit and each code used only once? Or the link can be sent as email too, using "basic admin notification" with {}-expression.

Near : gitlab.com/SondagesPro/SurveyAccess/surveyRegister