When migrating a survey, Sharegate displays the following error:
You are not allowed to respond again to this survey.
This probably happens because some users on the old site do not exist on the destination site. When trying to migrate those entries, Sharegate cannot submit entries as non-existing users, so it defaults to the user currently doing the migration (or to the "Unresolved users and groups" mapping if you set one). Then, when it tries to create more than one entry as this default user, SharePoint gives an error saying that the same user can't respond more than once to the survey.