Hi there, not having a go or trying to put the effort down, quite the contrary, a bit of criticism FWIW.
it is part of the data verification process and ensuring we don't get duplicates
Trying to use only cookies for that purpose is a (very) poor technical decision, it does not work like that.
It is important that we gather clean uncorrupted data that cant be questioned.
I'm sure you're at least partially aware of the limitations of online surveys. For the full picture, and if you want a sample that can stand up to scrutiny, I strongly suggest you should seek a competent statician for advice.
HTH.