It's Shauna here from the guru team, thanks for getting in touch. :)
By default, Shopify sets the same shipping and billing address to the same address on the checkout page. The checkbox that allows customers to have different shipping and billing addresses is still on the checkout page, so customers have the option to enable separate shipping and billing addresses.
There isn't a way to require that both shipping and billing addresses are the same within the admin, however I'd recommend a Fraud app such as Fraud Filter https://apps.shopify.com/fraud-filter where you can add this filter to orders, and block orders if the addresses don't match the criteria.
Hope this helps!
Has anyone figured this out? It is still not an option in Fraud filter. We also have lost $1,000's in fraudulent purchases due to this problem. We go in and cancel any orders that don't match billing/shipping because of this.. but it would be a lot easier if Shopify gave the option to require matching addresses if that's what our shop chooses in order to avoid lost dollars. Thanks!