First, the respondent is notified that their survey has been received and they are assigned a respondent id number.
Second, an "anti-ballot box stuffing" (ABBS) option is available on the survey administration page.
When ABBS is selected, cookies are sent to the respondent's computer when the first survey submit is received. The cookie contains information identifying the respondent and survey, and prevents them from retaking this particular survey.
If you are using Respondent Tracking mechanism there is a check in place to see if the particular email address has a completed response recorded. If there is a completed response for the particular email address then another response cannot be submitted.
Note that ABBS cannot distinguish between the survey owner's PC and the PC of an actual respondent. This means that if the ABBS option is in effect, you will only be allowed to complete or test the survey once. Therefore, do not activate ABBS until after the questionnaire is finalized and all testing has been done. Note that when ABBS is turned off, cookie checking is disabled and multiple responses are again permitted, even if a cookie has been placed on an individual's machine.