I see, in that case, if you have all of the user's information, then for each form against a user, save the unique codes.
Something like this:
User A -> 6AVS21AC
User B -> 9AS2OS1C
User A -> 8VPWAM3
This way, when each of the form is submitted, you'd know which user has submitted their form. You can also forbid submitting a form more than once using a flag or something similar. And if you need, you can also fetch and populate needed user's data.