There are two things to ensure here:
Disable cookies and ensure that the session length is set in such a way that the next respondent does not fall within the same session.
In the event that problems occur with the survey (respondents are booted out, entries are already pre-filled, etc.), please try to access the survey with SSL encryption (https:// rather than http:// in the survey link) to troubleshoot errors with an involved proxy server.
Optionally, you can uncheck the “Session ID in URL” box in the “Login options” tab. This results in a short survey link that does not contain any further details.