Good morning. And good question.
Wlhen someone revisits the survey they get an alert from Survey Monkey that they have already taken the survey, and is presented with no other options. There may be a "firewall" here that’s easy to work around that I haven’t tried yet. Also, it would be a simple task to revist the survey from another device and the survey would see that user as a "new participant". So, theoritically, you could participate in the same survey as a different user from your desktop, laptop, tablet, phone or your Samsung microwave oven app. There may be an industrial version out there that has face recognition, samples DNA or scans your record collection to see if you have any counterfit copies of the Led Zepplin IV album to reduce the likelinhood of fraud to close to zero, but I just wasn’t that ambitious. Maybe a PIN assingment or password might be something to look into -- next time?
Hopefully, the participants entered the exercise in good faith. That’s the best I could ask from a group of friends.