The check is already done in includes/class-wc-form-handler.php 843 and includes/class-wc-checkout.php 398
On the registration made during checkout caused the error:
PHP Notice: Undefined index: register in woocommerce/includes/wc-customer-functions.php on line 53
No perfect solution for #3162. DB locking would probably cause more
issues that its worth, we cannot reliably use transactions as some
clients still use MyIsam. Best to accept orders and deal with stock
conflicts after the fact.