In rare circumstances, you observe duplicate Survey records even though the 'Allow Duplicates' option is configured. The Survey records have created date/time plus or minus 1 minute.

The first step in preventing duplicates is to verify the 'Allow Duplicates' option is configured.  If it is not, you should configure it per the following documentation: Allow Duplicates.

If a survey recipient opens multiple links from the survey at nearly the same time, the Simple Survey page can load multiple times before each survey record is committed to the database. The query used to identify duplicates, therefore, returns no results, so duplicate records get created. Each transaction on the Salesforce platform is atomic, and Salesforce does not provide a mechanism to synchronize data between execution threads. This is a limitation of the Salesforce platform.