How to achieve, that a voter can only vote once on a quiz?

Thank you for your detailed analysis of this session cookie problem. Maybe, that this is also the bug, that the admin sometimes gets logged out from backend out of sudden.

It is not worth, to go deeper in it, because I assume, it will happen in relation with the old age of my websites. As I mentioned, I am going to renew these websites with Joomla 4.
 

I want to remind you kindly about my proposal from an earlier answer in this thread:

I have a suggestion for doing a pre check, wether the user already has completetd the quiz / survey:
Lets assume, the pre text option is set the way, that the text shows up before pushing the start button for the quiz / survey. Then we have to put the user detail fields before the start button. When he presses the start button, the content of the user details will be submitted and then can be checked against the user details in the quiz result list.
If double quizzing is detected, the user will see a popup window with the message, that he may not participate more than once.

Do you think, that could be a solution, which at least helps to keep most people from "double quizzing" – and of course it is not safe to prevent it.