The error message appears if the user the browser's (!) back button or use the reload page (F5) function of the browser.
We haven't had these problems while testing the survey.
Well, probably, you did not use the browser's back button ... give it a try :)
is there anything we could do now that the survey is active?
The warning is usually a good idea and therefore enabled by default. You should, however, be able to change the wording in "labels and text elements".