Avoid Duplicate respondents

One of the possible method is to create contact list trigger(update contact), which will update an embedded data (isSurveyCompleted) to 1 in the distributed contact list. At the start of the survey flow add branch logic and check - if isSurveyCompleted is equal to 1 then EOS with required message else continue with survey. Obviously, in order to do this we need to have a text entry email validated question at the start of survey only for anonymous link (use display logic on URL params ) with an authenticator (on distributed contact list) in survey flow. If the person gets authenticated, we will perform above mentioned logic else the respondent is new, we can continue with survey.

The only way I can think of is to ask the user's first name, last name, and date of birth in the survey, and after you collected the response then you consolidate the response based on a unique first name, last name, date of birth.