For anonymous users, the limit is enforced via a cookie that is placed on the client machine. If a respondent clears their cookies or uses a different browser then Checkbox will not have access to the cookie and can't enforce the limit.
The good news is that Checkbox 4.5 adds "Invitation Only" as a survey security option. This will require users to have a valid invitation to even take the survey and will allow enforcement of response limits on a per-invitation basis, regardless of whether a user is anonymous or not.
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