How do I stop people from signing up more than once?
Require a unique email address so each person can only respond once, and turn on a warning that flags anyone trying to sign up again.
Likely causes and how to fix them
Nothing ties a response to a single person
Require unique emails so the same address can't submit twice.
People resubmit by accident
Show a warning to returning respondents so they know they've already signed up, with a link to update their existing response instead of creating a new one.
One signup per person, with returning-respondent warnings. Require unique emails