Pranesh, I have the same issue. I can't prevent multiple entry, but I can detect it and act accordingly. I create a unique personalized variable. In the example you use, it looks like Student ID might work.
If you use send by email and the email address is the unique field, it needs to be an extra personalized variable in your survey, so your csv file will have the column "Email Address" which is reserved and used by CV to send the survey, plus another called "ControlEmail" which has the same value. (This is necessary because for some reason, the Email Address does not get reported in the survey if the survey is anonymous.)
When you run the survey, when Student 1 responds, your export file should have a field called 'Personalized date" and in your case it would look something like this for Student aaa:
First Name:aaa;Last Name:;Student ID:1;Gender:Male; etc.
If Student 1 clicks on the link you send him (with the email feature using personalized information stored in a csv) more than once (because "one survey per person" is turned off), each row should have the same value in the Personalized Data column and you can detect duplicates easily in Excel with Home > Conditional Formatting
Just for the record: I agree with you that one response per survey and anonymity should be unlinked. I have a similar issue with "Allow replies only from your organization" which is also linked to other options and should not be. I often survey both employees and retirees with the same survey (for example, we share the same health insurance plan) and employees are in the organization but retirees are not, and they are hardly anonymous.
Regards Bill Silverman
