We don’t currently offer a way to automatically prevent people answering a typeform more than once, but here are a couple of ways you can detect this.
1. Let respondents answer the survey as many times as they want and then download your results. In a spreadsheet application such as Google Sheets or Microsoft Excel, open your results and remove all duplicates coming from the same Network ID.
2. Embed the typeform in a page with cookies that does not allow the respondent to view the typeform more than once (this option will require someone with coding skills on your side).