App really has this logic for login via Google. In case your Store already has account connected to given email address our app can not use it to login via Social account. So for other Social networks you'll just get error message that you already have account created directly with email and password.
But Google support so called aliases. It means that you may add +<something> to your email address and it will still work properly. So our app creates another customer account for that alias email address. Customer will still be able to get notifications sent to that email address, so it will not affect your marketing.
And of course, if that email is NOT in your customer database yet, that suffix will NOT be added.