Duplicate customer records
No. The Webstore Auto Register SuiteExtender does not prevent duplicate customer records from being created. What it does do is to help prevent web customers creating duplicate customer records if they know they already are a customer but simply don't have a password to access the webstore. Without the Webstore Auto Register SuiteExtender existing customers who want to use your webstore for the first time would have no option other than to create a duplicate customer record or to contact you either telephonically or by email and request a password.