In this case, there are two aspects that have to be noticed:
1. Deactivate cookies and make sure, that the session-time is chosen so that the next participant won't slip into the same session.
2. In case you are encountering problems during your survey (participants are being kicked out, preallocated form fields etc.) please try using SSL-encryption (by using https:// in the survey link instead of http://)
to avoid errors caused by possibly involved proxy-servers.