When I set 'Multiple responses' setting to ‘Restrict to IP and Cookies’, some respondents who did not respond yet see a message that the respondents have already answered.

If you choose 'Restrict to IP and Cookies' for 'Multiple responses' setting, only one respondent among respondents who shares same IP address can answer. 

If you access the internet through the same routher at the office or cafe, all accessors may have the same IP address. In this environment, when one person completes an answer, the rest of the people, as they have the same IP address, can no longer answer.

In an environment where many respondents may have the same IP address, please do not use the 'Restrict to IP and Cookies' option.