We're having issues with respondents being mixed up. This seems to happen because some participants forward their unique survey url to other participants who for some reason or other can't find their own url.
Is it possible to setup the survey, such that each participant is asked to confirm either their name or email address before they are allowed to answer any questions in the survey?
If the survey in non-anonymous, you have access to the token data with placeholders like {TOKEN:TOKEN}, {TOKEN:FIRSTNAME}, {TOKEN:LASTNAME}. These can be used in a question validation equation.