A useful feature would be email based duplicate protection that could be used in conjunction with limiting access to a survey to respondents that received email invites.Consider the following scenario:

Currently, if I want to create a survey that

1. permits only invited respondents to participate

AND

2. allows only one response per invitation

AND

3. does not allow respondents to modify their answers once they complete the survey

I have two (equally bad ) options:

1. Activate Access Control and Quotas ->Email Invite respondents only, and set duplicate protection to Cookie Based. However, this will limit the number of responses per computer to one (this is may a problem if multiple respondents use the same computer)

2. Activate Access Control and Quotas ->Email Invite respondents only, disable duplicate protection and use one-time passwords to prevent respondents from re-taking the survey. The disadvanteage is that they couldn't come back to partially answered surveys, as their passwords would have expired.

The sgUID thing doesn't seem to be a solution in this case because:

1. its presence is not required, URL withous sgUID works
2. a malicious user may tamper with the URL and change sgUID to take the survey multiple times